diff --git a/header.go b/header.go
index ce2b832..57e5cb8 100644
--- a/header.go
+++ b/header.go
@@ -520,38 +520,8 @@ var ErrBadTrailer = errors.New("contain forbidden trailer")
 // 6. determining how to process the payload (e.g., Content-Encoding, Content-Type, Content-Range, and Trailer)
 //
 // Return ErrBadTrailer if contain any forbidden trailers.
-func (h *ResponseHeader) AddTrailerBytes(trailer []byte) error {
-	var err error
-	for i := -1; i+1 < len(trailer); {
-		trailer = trailer[i+1:]
-		i = bytes.IndexByte(trailer, ',')
-		if i < 0 {
-			i = len(trailer)
-		}
-		key := trailer[:i]
-		for len(key) > 0 && key[0] == ' ' {
-			key = key[1:]
-		}
-		for len(key) > 0 && key[len(key)-1] == ' ' {
-			key = key[:len(key)-1]
-		}
-		// Forbidden by RFC 7230, section 4.1.2
-		if isBadTrailer(key) {
-			err = ErrBadTrailer
-			continue
-		}
-		h.bufK = append(h.bufK[:0], key...)
-		normalizeHeaderKey(h.bufK, h.disableNormalizing)
-		if cap(h.trailer) > len(h.trailer) {
-			h.trailer = h.trailer[:len(h.trailer)+1]
-			h.trailer[len(h.trailer)-1] = append(h.trailer[len(h.trailer)-1][:0], h.bufK...)
-		} else {
-			key = make([]byte, len(h.bufK))
-			copy(key, h.bufK)
-			h.trailer = append(h.trailer, key)
-		}
-	}
-
+func (h *ResponseHeader) AddTrailerBytes(trailer []byte) (err error) {
+	h.bufK, h.trailer, err = addTrailerBytes(trailer, h.bufK, h.trailer, h.disableNormalizing)
 	return err
 }
 
@@ -875,38 +845,8 @@ func (h *RequestHeader) AddTrailer(trailer string) error {
 // 6. determining how to process the payload (e.g., Content-Encoding, Content-Type, Content-Range, and Trailer)
 //
 // Return ErrBadTrailer if contain any forbidden trailers.
-func (h *RequestHeader) AddTrailerBytes(trailer []byte) error {
-	var err error
-	for i := -1; i+1 < len(trailer); {
-		trailer = trailer[i+1:]
-		i = bytes.IndexByte(trailer, ',')
-		if i < 0 {
-			i = len(trailer)
-		}
-		key := trailer[:i]
-		for len(key) > 0 && key[0] == ' ' {
-			key = key[1:]
-		}
-		for len(key) > 0 && key[len(key)-1] == ' ' {
-			key = key[:len(key)-1]
-		}
-		// Forbidden by RFC 7230, section 4.1.2
-		if isBadTrailer(key) {
-			err = ErrBadTrailer
-			continue
-		}
-		h.bufK = append(h.bufK[:0], key...)
-		normalizeHeaderKey(h.bufK, h.disableNormalizing)
-		if cap(h.trailer) > len(h.trailer) {
-			h.trailer = h.trailer[:len(h.trailer)+1]
-			h.trailer[len(h.trailer)-1] = append(h.trailer[len(h.trailer)-1][:0], h.bufK...)
-		} else {
-			key = make([]byte, len(h.bufK))
-			copy(key, h.bufK)
-			h.trailer = append(h.trailer, key)
-		}
-	}
-
+func (h *RequestHeader) AddTrailerBytes(trailer []byte) (err error) {
+	h.bufK, h.trailer, err = addTrailerBytes(trailer, h.bufK, h.trailer, h.disableNormalizing)
 	return err
 }
 
@@ -1321,8 +1261,8 @@ func (h *RequestHeader) VisitAll(f func(key, value []byte)) {
 func (h *RequestHeader) VisitAllInOrder(f func(key, value []byte)) {
 	var s headerScanner
 	s.b = h.rawHeaders
-	s.disableNormalizing = h.disableNormalizing
 	for s.next() {
+		normalizeHeaderKey(s.key, h.disableNormalizing || bytes.IndexByte(s.key, ' ') != -1)
 		if len(s.key) > 0 {
 			f(s.key, s.value)
 		}
@@ -1338,7 +1278,7 @@ func (h *ResponseHeader) Del(key string) {
 // DelBytes deletes header with the given key.
 func (h *ResponseHeader) DelBytes(key []byte) {
 	h.bufK = append(h.bufK[:0], key...)
-	normalizeHeaderKey(h.bufK, h.disableNormalizing)
+	normalizeHeaderKey(h.bufK, h.disableNormalizing || bytes.IndexByte(key, ' ') != -1)
 	h.del(h.bufK)
 }
 
@@ -1372,7 +1312,7 @@ func (h *RequestHeader) Del(key string) {
 // DelBytes deletes header with the given key.
 func (h *RequestHeader) DelBytes(key []byte) {
 	h.bufK = append(h.bufK[:0], key...)
-	normalizeHeaderKey(h.bufK, h.disableNormalizing)
+	normalizeHeaderKey(h.bufK, h.disableNormalizing || bytes.IndexByte(key, ' ') != -1)
 	h.del(h.bufK)
 }
 
@@ -1638,7 +1578,7 @@ func (h *ResponseHeader) SetBytesV(key string, value []byte) {
 // Use AddBytesKV for setting multiple header values under the same key.
 func (h *ResponseHeader) SetBytesKV(key, value []byte) {
 	h.bufK = append(h.bufK[:0], key...)
-	normalizeHeaderKey(h.bufK, h.disableNormalizing)
+	normalizeHeaderKey(h.bufK, h.disableNormalizing || bytes.IndexByte(key, ' ') != -1)
 	h.SetCanonical(h.bufK, value)
 }
 
@@ -1864,7 +1804,7 @@ func (h *RequestHeader) SetBytesV(key string, value []byte) {
 // Use AddBytesKV for setting multiple header values under the same key.
 func (h *RequestHeader) SetBytesKV(key, value []byte) {
 	h.bufK = append(h.bufK[:0], key...)
-	normalizeHeaderKey(h.bufK, h.disableNormalizing)
+	normalizeHeaderKey(h.bufK, h.disableNormalizing || bytes.IndexByte(key, ' ') != -1)
 	h.SetCanonical(h.bufK, value)
 }
 
@@ -1900,7 +1840,7 @@ func (h *ResponseHeader) Peek(key string) []byte {
 // Do not store references to returned value. Make copies instead.
 func (h *ResponseHeader) PeekBytes(key []byte) []byte {
 	h.bufK = append(h.bufK[:0], key...)
-	normalizeHeaderKey(h.bufK, h.disableNormalizing)
+	normalizeHeaderKey(h.bufK, h.disableNormalizing || bytes.IndexByte(key, ' ') != -1)
 	return h.peek(h.bufK)
 }
 
@@ -1921,7 +1861,7 @@ func (h *RequestHeader) Peek(key string) []byte {
 // Do not store references to returned value. Make copies instead.
 func (h *RequestHeader) PeekBytes(key []byte) []byte {
 	h.bufK = append(h.bufK[:0], key...)
-	normalizeHeaderKey(h.bufK, h.disableNormalizing)
+	normalizeHeaderKey(h.bufK, h.disableNormalizing || bytes.IndexByte(key, ' ') != -1)
 	return h.peek(h.bufK)
 }
 
@@ -2239,7 +2179,8 @@ func (h *ResponseHeader) tryReadTrailer(r *bufio.Reader, n int) error {
 		return fmt.Errorf("error when reading response trailer: %w", err)
 	}
 	b = mustPeekBuffered(r)
-	headersLen, errParse := h.parseTrailer(b)
+	trailers, headersLen, errParse := parseTrailer(b, h.h, h.disableNormalizing)
+	h.h = trailers
 	if errParse != nil {
 		if err == io.EOF {
 			return err
@@ -2348,7 +2289,8 @@ func (h *RequestHeader) tryReadTrailer(r *bufio.Reader, n int) error {
 		return fmt.Errorf("error when reading request trailer: %w", err)
 	}
 	b = mustPeekBuffered(r)
-	headersLen, errParse := h.parseTrailer(b)
+	trailers, headersLen, errParse := parseTrailer(b, h.h, h.disableNormalizing)
+	h.h = trailers
 	if errParse != nil {
 		if err == io.EOF {
 			return err
@@ -2725,43 +2667,6 @@ func (h *ResponseHeader) parse(buf []byte) (int, error) {
 	return m + n, nil
 }
 
-func (h *ResponseHeader) parseTrailer(buf []byte) (int, error) {
-	// Skip any 0 length chunk.
-	if buf[0] == '0' {
-		skip := len(strCRLF) + 1
-		if len(buf) < skip {
-			return 0, io.EOF
-		}
-		buf = buf[skip:]
-	}
-
-	var s headerScanner
-	s.b = buf
-	s.disableNormalizing = h.disableNormalizing
-	var err error
-	for s.next() {
-		if len(s.key) > 0 {
-			if bytes.IndexByte(s.key, ' ') != -1 || bytes.IndexByte(s.key, '\t') != -1 {
-				err = fmt.Errorf("invalid trailer key %q", s.key)
-				continue
-			}
-			// Forbidden by RFC 7230, section 4.1.2
-			if isBadTrailer(s.key) {
-				err = fmt.Errorf("forbidden trailer key %q", s.key)
-				continue
-			}
-			h.h = appendArgBytes(h.h, s.key, s.value, argsHasValue)
-		}
-	}
-	if s.err != nil {
-		return 0, s.err
-	}
-	if err != nil {
-		return 0, err
-	}
-	return s.hLen, nil
-}
-
 func (h *RequestHeader) ignoreBody() bool {
 	return h.IsGet() || h.IsHead()
 }
@@ -2784,41 +2689,82 @@ func (h *RequestHeader) parse(buf []byte) (int, error) {
 	return m + n, nil
 }
 
-func (h *RequestHeader) parseTrailer(buf []byte) (int, error) {
-	// Skip any 0 length chunk.
-	if buf[0] == '0' {
-		skip := len(strCRLF) + 1
-		if len(buf) < skip {
-			return 0, io.EOF
+func addTrailerBytes(src, buf []byte, trailers [][]byte, disableNormalizing bool) ([]byte, [][]byte, error) {
+	var err error
+	for i := -1; i+1 < len(src); {
+		src = src[i+1:]
+		i = bytes.IndexByte(src, ',')
+		if i < 0 {
+			i = len(src)
 		}
-		buf = buf[skip:]
+		key := src[:i]
+		for len(key) > 0 && key[0] == ' ' {
+			key = key[1:]
+		}
+		for len(key) > 0 && key[len(key)-1] == ' ' {
+			key = key[:len(key)-1]
+		}
+		// Forbidden by RFC 7230, section 4.1.2
+		if isBadTrailer(key) {
+			err = ErrBadTrailer
+			continue
+		}
+		buf = append(buf[:0], key...)
+		normalizeHeaderKey(buf, disableNormalizing || bytes.IndexByte(buf, ' ') != -1)
+		if cap(trailers) > len(trailers) {
+			trailers = trailers[:len(trailers)+1]
+			trailers[len(trailers)-1] = append(trailers[len(trailers)-1][:0], buf...)
+		} else {
+			key = make([]byte, len(buf))
+			copy(key, buf)
+			trailers = append(trailers, key)
+		}
+	}
+
+	return buf, trailers, err
+}
+
+func parseTrailer(src []byte, dest []argsKV, disableNormalizing bool) ([]argsKV, int, error) {
+	// Skip any 0 length chunk.
+	if src[0] == '0' {
+		skip := len(strCRLF) + 1
+		if len(src) < skip {
+			return dest, 0, io.EOF
+		}
+		src = src[skip:]
 	}
 
 	var s headerScanner
-	s.b = buf
-	s.disableNormalizing = h.disableNormalizing
-	var err error
+	s.b = src
+
 	for s.next() {
-		if len(s.key) > 0 {
-			if bytes.IndexByte(s.key, ' ') != -1 || bytes.IndexByte(s.key, '\t') != -1 {
-				err = fmt.Errorf("invalid trailer key %q", s.key)
-				continue
-			}
-			// Forbidden by RFC 7230, section 4.1.2
-			if isBadTrailer(s.key) {
-				err = fmt.Errorf("forbidden trailer key %q", s.key)
-				continue
-			}
-			h.h = appendArgBytes(h.h, s.key, s.value, argsHasValue)
+		if len(s.key) == 0 {
+			continue
 		}
+		disable := disableNormalizing
+		for _, ch := range s.key {
+			if !validHeaderFieldByte(ch) {
+				// We accept invalid headers with a space before the
+				// colon, but must not canonicalize them.
+				// See: https://github.com/valyala/fasthttp/issues/1917
+				if ch == ' ' {
+					disable = true
+					continue
+				}
+				return dest, 0, fmt.Errorf("invalid trailer key %q", s.key)
+			}
+		}
+		// Forbidden by RFC 7230, section 4.1.2
+		if isBadTrailer(s.key) {
+			return dest, 0, fmt.Errorf("forbidden trailer key %q", s.key)
+		}
+		normalizeHeaderKey(s.key, disable)
+		dest = appendArgBytes(dest, s.key, s.value, argsHasValue)
 	}
 	if s.err != nil {
-		return 0, s.err
+		return dest, 0, s.err
 	}
-	if err != nil {
-		return 0, err
-	}
-	return s.hLen, nil
+	return dest, s.hLen, nil
 }
 
 func isBadTrailer(key []byte) bool {
@@ -3019,7 +2965,6 @@ func (h *ResponseHeader) parseHeaders(buf []byte) (int, error) {
 
 	var s headerScanner
 	s.b = buf
-	s.disableNormalizing = h.disableNormalizing
 	var kv *argsKV
 
 	for s.next() {
@@ -3028,12 +2973,22 @@ func (h *ResponseHeader) parseHeaders(buf []byte) (int, error) {
 			return 0, fmt.Errorf("invalid header key %q", s.key)
 		}
 
+		disableNormalizing := h.disableNormalizing
 		for _, ch := range s.key {
 			if !validHeaderFieldByte(ch) {
 				h.connectionClose = true
+				// We accept invalid headers with a space before the
+				// colon, but must not canonicalize them.
+				// See: https://github.com/valyala/fasthttp/issues/1917
+				if ch == ' ' {
+					disableNormalizing = true
+					continue
+				}
 				return 0, fmt.Errorf("invalid header key %q", s.key)
 			}
 		}
+		normalizeHeaderKey(s.key, disableNormalizing)
+
 		for _, ch := range s.value {
 			if !validHeaderValueByte(ch) {
 				h.connectionClose = true
@@ -3136,7 +3091,6 @@ func (h *RequestHeader) parseHeaders(buf []byte) (int, error) {
 
 	var s headerScanner
 	s.b = buf
-	s.disableNormalizing = h.disableNormalizing
 
 	for s.next() {
 		if len(s.key) == 0 {
@@ -3144,12 +3098,19 @@ func (h *RequestHeader) parseHeaders(buf []byte) (int, error) {
 			return 0, fmt.Errorf("invalid header key %q", s.key)
 		}
 
+		disableNormalizing := h.disableNormalizing
 		for _, ch := range s.key {
 			if !validHeaderFieldByte(ch) {
+				if ch == ' ' {
+					disableNormalizing = true
+					continue
+				}
 				h.connectionClose = true
 				return 0, fmt.Errorf("invalid header key %q", s.key)
 			}
 		}
+		normalizeHeaderKey(s.key, disableNormalizing)
+
 		for _, ch := range s.value {
 			if !validHeaderValueByte(ch) {
 				h.connectionClose = true
@@ -3304,8 +3265,7 @@ type headerScanner struct {
 	nextColon   int
 	nextNewLine int
 
-	disableNormalizing bool
-	initialized        bool
+	initialized bool
 }
 
 func (s *headerScanner) next() bool {
@@ -3351,7 +3311,6 @@ func (s *headerScanner) next() bool {
 		return false
 	}
 	s.key = s.b[:n]
-	normalizeHeaderKey(s.key, s.disableNormalizing)
 	n++
 	for len(s.b) > n && (s.b[n] == ' ' || s.b[n] == '\t') {
 		n++
@@ -3482,7 +3441,7 @@ func initHeaderKV(bufK, bufV []byte, key, value string, disableNormalizing bool)
 
 func getHeaderKeyBytes(bufK []byte, key string, disableNormalizing bool) []byte {
 	bufK = append(bufK[:0], key...)
-	normalizeHeaderKey(bufK, disableNormalizing)
+	normalizeHeaderKey(bufK, disableNormalizing || bytes.IndexByte(bufK, ' ') != -1)
 	return bufK
 }
 
diff --git a/header_test.go b/header_test.go
index 1af2631..3c3affa 100644
--- a/header_test.go
+++ b/header_test.go
@@ -327,6 +327,7 @@ func TestRequestRawHeaders(t *testing.T) {
 
 	kvs := "hOsT: foobar\r\n" +
 		"value:  b\r\n" +
+		"uSeR agent: agent\r\n" +
 		"\r\n"
 	t.Run("normalized", func(t *testing.T) {
 		s := "GET / HTTP/1.1\r\n" + kvs
@@ -343,6 +344,12 @@ func TestRequestRawHeaders(t *testing.T) {
 		if !bytes.Equal(v2, []byte{'b'}) {
 			t.Fatalf("expecting non empty value. Got %q", v2)
 		}
+		// We accept invalid headers with a space.
+		// See: https://github.com/valyala/fasthttp/issues/1917
+		v3 := h.Peek("uSeR agent")
+		if !bytes.Equal(v3, []byte("agent")) {
+			t.Fatalf("expecting non empty value. Got %q", v3)
+		}
 		if raw := h.RawHeaders(); string(raw) != exp {
 			t.Fatalf("expected header %q, got %q", exp, raw)
 		}
@@ -1860,8 +1867,8 @@ func TestResponseHeaderAddTrailerError(t *testing.T) {
 	t.Parallel()
 
 	var h ResponseHeader
-	err := h.AddTrailer("Foo,   Content-Length , Bar,Transfer-Encoding,")
-	expectedTrailer := "Foo, Bar"
+	err := h.AddTrailer("Foo,   Content-Length , bAr,Transfer-Encoding, uSer aGent")
+	expectedTrailer := "Foo, Bar, uSer aGent"
 
 	if !errors.Is(err, ErrBadTrailer) {
 		t.Fatalf("unexpected err %q. Expected %q", err, ErrBadTrailer)
diff --git a/server_test.go b/server_test.go
index 59f27d1..c0ab2ce 100644
--- a/server_test.go
+++ b/server_test.go
@@ -189,8 +189,8 @@ func TestServerInvalidHeader(t *testing.T) {
 
 	s := &Server{
 		Handler: func(ctx *RequestCtx) {
-			if ctx.Request.Header.Peek("Foo") != nil || ctx.Request.Header.Peek("Foo ") != nil {
-				t.Error("expected Foo header")
+			if ctx.Request.Header.Peek("Foő") != nil || ctx.Request.Header.Peek("Foő ") != nil {
+				t.Error("expected Foő header")
 			}
 		},
 		Logger: &testLogger{},
@@ -208,7 +208,7 @@ func TestServerInvalidHeader(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
-	if _, err = c.Write([]byte("POST /foo HTTP/1.1\r\nHost: gle.com\r\nFoo : bar\r\nContent-Length: 5\r\n\r\n12345")); err != nil {
+	if _, err = c.Write([]byte("POST /foo HTTP/1.1\r\nHost: gle.com\r\nFoő : bar\r\nContent-Length: 5\r\n\r\n12345")); err != nil {
 		t.Fatal(err)
 	}
 
@@ -225,7 +225,7 @@ func TestServerInvalidHeader(t *testing.T) {
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
-	if _, err = c.Write([]byte("GET /foo HTTP/1.1\r\nHost: gle.com\r\nFoo : bar\r\n\r\n")); err != nil {
+	if _, err = c.Write([]byte("GET /foo HTTP/1.1\r\nHost: gle.com\r\nFoő : bar\r\n\r\n")); err != nil {
 		t.Fatal(err)
 	}