diff --git a/acorn.js b/acorn.js
index f966917221..436a156ff7 100644
--- a/acorn.js
+++ b/acorn.js
@@ -834,7 +834,7 @@
         ch = input.charCodeAt(++tokPos);
         var octal = /^[0-7]+/.exec(input.slice(tokPos, tokPos + 3));
         if (octal) octal = octal[0];
-        while (octal && parseInt(octal, 8) > 255) octal = octal.slice(0, octal.length - 1);
+        while (octal && parseInt(octal, 8) > 255) octal = octal.slice(0, -1);
         if (octal === "0") octal = null;
         ++tokPos;
         if (octal) {