diff --git a/build/build.py b/build/build.py
index cbeb1e6e3..3f8c4ef54 100755
--- a/build/build.py
+++ b/build/build.py
@@ -71,7 +71,8 @@ common_closure_opts = [
      shakaBuildHelpers.cygwin_safe_path(shakaBuildHelpers.get_source_base())),
 
     '--generate_exports',
-
+]
+common_closure_defines = [
     '-D', 'COMPILED=true',
     '-D', 'goog.STRICT_MODE_COMPATIBLE=true',
     '-D', 'goog.ENABLE_DEBUG_LOADER=false',
@@ -80,8 +81,9 @@ common_closure_opts = [
 debug_closure_opts = [
     # Don't use a wrapper script in debug mode so all the internals are visible
     # on the global object.
-
     '-O', 'SIMPLE',
+]
+debug_closure_defines = [
     '-D', 'goog.DEBUG=true',
     '-D', 'goog.asserts.ENABLE_ASSERTS=true',
     '-D', 'shaka.log.MAX_LOG_LEVEL=4',  # shaka.log.Level.DEBUG
@@ -89,8 +91,9 @@ debug_closure_opts = [
 release_closure_opts = [
     ('--output_wrapper_file=%s/build/wrapper.template.js' %
      shakaBuildHelpers.cygwin_safe_path(shakaBuildHelpers.get_source_base())),
-
     '-O', 'ADVANCED',
+]
+release_closure_defines = [
     '-D', 'goog.DEBUG=false',
     '-D', 'goog.asserts.ENABLE_ASSERTS=false',
     '-D', 'shaka.log.MAX_LOG_LEVEL=0',
@@ -233,12 +236,11 @@ class Build(object):
 
     return True
 
-  def build_raw(self, extra_opts, is_debug):
+  def build_raw(self, closure_opts):
     """Builds the files in |self.include| using the given extra Closure options.
 
     Args:
-      extra_opts: An array of extra options to give to Closure.
-      is_debug: True to compile for debugging, false for release.
+      closure_opts: An array of options to give to Closure.
 
     Returns:
       True on success; False on failure.
@@ -249,12 +251,7 @@ class Build(object):
     files = [shakaBuildHelpers.cygwin_safe_path(f) for f in self.include]
     files.sort()
 
-    if is_debug:
-      closure_opts = common_closure_opts + debug_closure_opts
-    else:
-      closure_opts = common_closure_opts + release_closure_opts
-
-    cmd_line = ['java', '-jar', jar] + closure_opts + extra_opts + files
+    cmd_line = ['java', '-jar', jar] + closure_opts + files
     if shakaBuildHelpers.execute_get_code(cmd_line) != 0:
       logging.error('Build failed')
       return False
@@ -325,9 +322,17 @@ class Build(object):
                           'to override.')
           return True
 
-    opts = ['--create_source_map', result_map, '--js_output_file', result_file,
-            '--source_map_location_mapping', source_base + '|..']
-    if not self.build_raw(opts, is_debug):
+    closure_opts = common_closure_opts + common_closure_defines
+    if is_debug:
+      closure_opts += debug_closure_opts + debug_closure_defines
+    else:
+      closure_opts += release_closure_opts + release_closure_defines
+
+    closure_opts += [
+        '--create_source_map', result_map, '--js_output_file', result_file,
+        '--source_map_location_mapping', source_base + '|..'
+    ]
+    if not self.build_raw(closure_opts):
       return False
 
     # Add a special source-mapping comment so that Chrome and Firefox can map
diff --git a/build/check.py b/build/check.py
index 830bb1385..fef2ef7b2 100755
--- a/build/check.py
+++ b/build/check.py
@@ -128,11 +128,63 @@ def check_tests():
            get('third_party', 'closure'))
   test_build = build.Build(set(files))
 
+  closure_opts = build.common_closure_opts + build.common_closure_defines
+  closure_opts += build.debug_closure_opts + build.debug_closure_defines
+
   # Ignore missing goog.require since we assume the whole library is
   # already included.
-  opts = ['--jscomp_off=missingRequire', '--jscomp_off=strictMissingRequire',
-          '--checks-only', '-O', 'SIMPLE']
-  return test_build.build_raw(opts, is_debug=True)
+  closure_opts += [
+      '--jscomp_off=missingRequire', '--jscomp_off=strictMissingRequire',
+      '--checks-only', '-O', 'SIMPLE'
+  ]
+  return test_build.build_raw(closure_opts)
+
+
+def check_externs():
+  """Runs an extra compile pass over the generated externs to ensure that they
+  are usable.
+
+  Returns:
+    True on success, False on failure.
+  """
+  logging.info('Checking the usability of generated externs...')
+
+  # Create a complete "build" object.
+  externs_build = build.Build()
+  if not externs_build.parse_build(['+@complete'], os.getcwd()):
+    return False
+  externs_build.add_core()
+
+  # Use it to generate externs for the next check.
+  if not externs_build.generate_externs('check'):
+    return False
+
+  # Create a custom "build" object, add all manually-written externs, then add
+  # the generated externs we just generated.
+  source_base = shakaBuildHelpers.get_source_base()
+  manual_externs = shakaBuildHelpers.get_all_files(
+      os.path.join(source_base, 'externs'), re.compile(r'.*\.js$'))
+  generated_externs = os.path.join(
+      source_base, 'dist', 'shaka-player.check.externs.js')
+
+  check_build = build.Build()
+  check_build.include = set(manual_externs)
+  check_build.include.add(generated_externs)
+
+  # Build with the complete set of externs, but without any application code.
+  # This will help find issues in the generated externs, independent of the app.
+  # Since we have no app, don't use the defines.  Unused defines cause a
+  # compilation error.
+  closure_opts = build.common_closure_opts + build.debug_closure_opts + [
+      '--checks-only', '-O', 'SIMPLE'
+  ]
+  ok = check_build.build_raw(closure_opts)
+
+  # Clean up the temporary externs we just generated.
+  os.unlink(generated_externs)
+
+  # Return the success/failure of the build above.
+  return ok
 
 
 def usage():
@@ -151,16 +203,17 @@ def main(args):
       usage()
       return 1
 
-  if not check_lint():
-    return 1
-  elif not check_html_lint():
-    return 1
-  elif not check_complete():
-    return 1
-  elif not check_tests():
-    return 1
-  else:
-    return 0
+  steps = [
+    check_lint,
+    check_html_lint,
+    check_complete,
+    check_tests,
+    check_externs,
+  ]
+  for step in steps:
+    if not step():
+      return 1
+  return 0
 
 
 if __name__ == '__main__':