diff --git a/src/ng/filter/filter.js b/src/ng/filter/filter.js
index a0b49948d55c..acecd9244c2e 100644
--- a/src/ng/filter/filter.js
+++ b/src/ng/filter/filter.js
@@ -226,7 +226,9 @@ function deepCompare(actual, expected, comparator, anyPropertyKey, matchAgainstA
       var key;
       if (matchAgainstAnyProp) {
         for (key in actual) {
-          if ((key.charAt(0) !== '$') && deepCompare(actual[key], expected, comparator, anyPropertyKey, true)) {
+          // Under certain, rare, circumstances, key may not be a string and `charAt` will be undefined
+          // See: https://github.com/angular/angular.js/issues/15644
+          if (key.charAt && (key.charAt(0) !== '$') && deepCompare(actual[key], expected, comparator, anyPropertyKey, true)) {
             return true;
           }
         }