diff --git a/src/tosource.ts b/src/tosource.ts
index 6d5f574..245b23c 100644
--- a/src/tosource.ts
+++ b/src/tosource.ts
@@ -107,6 +107,6 @@ const KEYWORD_REGEXP =
 
 function legalKey(key: string) {
   return (
-    /^([a-z_$][0-9a-z_$]*|[0-9]+)$/gi.test(key) && !KEYWORD_REGEXP.test(key)
+    /^([a-z_$][0-9a-z_$]*|0|[1-9][0-9]*)$/gi.test(key) && !KEYWORD_REGEXP.test(key)
   );
 }