diff --git a/acorn.js b/acorn.js
index d17a29c418..f5e9d835e3 100644
--- a/acorn.js
+++ b/acorn.js
@@ -164,6 +164,9 @@
         comments.push(comment);
       };
     }
+    if (opts.strictMode) {
+      strict = true;
+    }
     isKeyword = options.ecmaVersion >= 6 ? isEcma6Keyword : isEcma5AndLessKeyword;
   }