diff --git a/js/src/jsscript.cpp b/js/src/jsscript.cpp
index 80e4a9ab621b..595469cae5bc 100644
--- a/js/src/jsscript.cpp
+++ b/js/src/jsscript.cpp
@@ -2216,6 +2216,7 @@ ScriptSource::~ScriptSource()
         }
     };
 
+    MOZ_ASSERT(refs == 0);
     MOZ_ASSERT_IF(inCompressedSourceSet, data.is<Compressed>());
 
     DestroyMatcher dm(*this);