--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -12550,17 +12550,19 @@ class CGDictionary(CGThing):
         we can avoid atomizing strings all the time, then we just spit
         out the getMemberConversion() output for each member,
         separated by newlines.
 
         """
         body = dedent("""
             // Passing a null JSContext is OK only if we're initing from null,
             // Since in that case we will not have to do any property gets
-            MOZ_ASSERT_IF(!cx, val.isNull());
+            // Also evaluate isNullOrUndefined in order to avoid false-positive
+            // checkers by static analysis tools
+            MOZ_ASSERT_IF(!cx, val.isNull() && val.isNullOrUndefined());
             """)
 
         if self.needToInitIds:
             body += fill(
                 """
                 ${dictName}Atoms* atomsCache = nullptr;
                 if (cx) {
                   atomsCache = GetAtomCache<${dictName}Atoms>(cx);