diff --git a/lodash.src.js b/lodash.src.js
index a47c268ba..26fa492a0 100644
--- a/lodash.src.js
+++ b/lodash.src.js
@@ -4224,7 +4224,7 @@
      * @returns {boolean} Returns `true` if `value` is a valid index, else `false`.
      */
     function isIndex(value, length) {
-      value = (typeof value == 'number' || reIsUint.test(value)) ? +value : NaN;
+      value = (typeof value == 'number' || reIsUint.test(value)) ? +value : -1;
       length = length == null ? MAX_SAFE_INTEGER : length;
       return value > -1 && value % 1 == 0 && value < length;
     }