--- a/netwerk/protocol/http/AltDataOutputStreamParent.cpp
+++ b/netwerk/protocol/http/AltDataOutputStreamParent.cpp
@@ -34,17 +34,17 @@ AltDataOutputStreamParent::RecvWriteData
       Unused << SendError(mStatus);
     }
     return IPC_OK();
   }
   nsresult rv;
   uint32_t n;
   if (mOutputStream) {
     rv = mOutputStream->Write(data.BeginReading(), data.Length(), &n);
-    MOZ_ASSERT(n == data.Length());
+    MOZ_ASSERT(n == data.Length() || NS_FAILED(rv));
     if (NS_FAILED(rv) && mIPCOpen) {
       Unused << SendError(rv);
     }
   }
   return IPC_OK();
 }
 
 mozilla::ipc::IPCResult