diff --git a/lib/core/Prism.js b/lib/core/Prism.js
index 3030745466..b75f2834e6 100644
--- a/lib/core/Prism.js
+++ b/lib/core/Prism.js
@@ -256,9 +256,9 @@ const Prism = {
 
     const strarr = [text];
 
-    const rest = grammar.rest;
 
-    if (rest) {
+    if (grammar && grammar.rest) {
+      const rest = grammar.rest;
       for (var token in rest) {
         grammar[token] = rest[token];
       }