diff --git a/js/src/vm/StructuredClone.cpp b/js/src/vm/StructuredClone.cpp
index 25e9e93b3b..79f2d23d96 100644
--- a/js/src/vm/StructuredClone.cpp
+++ b/js/src/vm/StructuredClone.cpp
@@ -1700,6 +1700,8 @@ JSStructuredCloneWriter::write(HandleValue v)
                 return false;
 
             if (cls == ESClass::Map) {
+                if (!counts.back())
+                    return false;
                 counts.back()--;
                 RootedValue val(context(), entries.back());
                 entries.popBack();