diff --git a/lib/drm/drm_engine.js b/lib/drm/drm_engine.js
index 44b877e2d..5a8157a0b 100644
--- a/lib/drm/drm_engine.js
+++ b/lib/drm/drm_engine.js
@@ -1484,11 +1484,12 @@ shaka.drm.DrmEngine = class {
       await session.update(response.data);
     } catch (error) {
       // Session update failed!
+      const errorMessage = (error && error.message) || String(error);
       const shakaErr = new shaka.util.Error(
           shaka.util.Error.Severity.CRITICAL,
           shaka.util.Error.Category.DRM,
           shaka.util.Error.Code.LICENSE_RESPONSE_REJECTED,
-          error.message);
+          errorMessage);
       this.onError_(shakaErr);
       if (metadata && metadata.updatePromise) {
         metadata.updatePromise.reject(shakaErr);