diff --git a/dom/base/BodyStream.cpp b/dom/base/BodyStream.cpp
index 0db949ca3c8e..6760e6fe9220 100644
--- a/dom/base/BodyStream.cpp
+++ b/dom/base/BodyStream.cpp
@@ -350,11 +350,11 @@ BodyStream::OnInputStreamReady(nsIAsyncInputStream* aStream) {
 
   {
     MutexAutoUnlock unlock(mMutex);
-    JS::ReadableStreamUpdateDataAvailableFromSource(cx, stream, size);
-  }
+    bool ok = JS::ReadableStreamUpdateDataAvailableFromSource(cx, stream, size);
 
-  // The WriteInto callback changes mState to eChecking.
-  MOZ_DIAGNOSTIC_ASSERT(mState == eChecking);
+    // The WriteInto callback changes mState to eChecking.
+    MOZ_DIAGNOSTIC_ASSERT_IF(ok, mState == eChecking);
+  }
 
   return NS_OK;
 }