diff --git a/lib/assert/macros.js b/lib/assert/macros.js
index b91449d..494ad6d 100644
--- a/lib/assert/macros.js
+++ b/lib/assert/macros.js
@@ -70,7 +70,7 @@ assert.lesser = function (actual, expected, message) {
 assert.inDelta = function (actual, expected, delta, message) {
     var lower = expected - delta;
     var upper = expected + delta;
-    if (actual < lower || actual > upper) {
+    if (actual != +actual || actual < lower || actual > upper) {
         assert.fail(actual, expected, message || "expected {actual} to be in within *" + delta.toString() + "* of {expected}", null, assert.inDelta);
     }
 };