--- a/image/ImageOps.cpp
+++ b/image/ImageOps.cpp
@@ -108,17 +108,20 @@ ImageOps::DecodeToSurface(nsIInputStream
 
   // Write the data into a SourceBuffer.
   NotNull<RefPtr<SourceBuffer>> sourceBuffer = WrapNotNull(new SourceBuffer());
   sourceBuffer->ExpectLength(length);
   rv = sourceBuffer->AppendFromInputStream(inputStream, length);
   if (NS_FAILED(rv)) {
     return nullptr;
   }
-  sourceBuffer->Complete(NS_OK);
+  // Make sure our sourceBuffer is marked as complete.
+  if (!sourceBuffer->IsComplete()) {
+    sourceBuffer->Complete(NS_OK);
+  }
 
   // Create a decoder.
   DecoderType decoderType =
     DecoderFactory::GetDecoderType(PromiseFlatCString(aMimeType).get());
   RefPtr<Decoder> decoder =
     DecoderFactory::CreateAnonymousDecoder(decoderType, sourceBuffer,
                                            Nothing(), ToSurfaceFlags(aFlags));
   if (!decoder) {