diff --git a/grudge/models/wave.py b/grudge/models/wave.py
index 8f6cc76f79f124b40574f9d24f13ee4d54b6cb2b..a8b5470070c528fe681bd13702390f1ab58fa498 100644
--- a/grudge/models/wave.py
+++ b/grudge/models/wave.py
@@ -164,7 +164,7 @@ class WeakWaveOperator(HyperbolicOperator):
             )
         )
 
-        result[0] += self.source_f(actx, dcoll, t)
+        result[0] = result[0] + self.source_f(actx, dcoll, t)
 
         return result