diff --git a/Foundation/CPIndexSet.j b/Foundation/CPIndexSet.j
index 33ecfbdf7..ffa8300f2 100644
--- a/Foundation/CPIndexSet.j
+++ b/Foundation/CPIndexSet.j
@@ -21,6 +21,7 @@
  */
 
 #include "CPRange.h"
+#include "Foundation.h"
 
 @import "CPArray.j"
 @import "CPObject.j"
@@ -79,6 +80,10 @@
 */
 - (id)initWithIndex:(CPInteger)anIndex
 {
+    if (!_IS_NUMERIC(anIndex))
+        [CPException raise:CPInvalidArgumentException
+                    reason:"Invalid index"];
+
     return [self initWithIndexesInRange:_CPMakeRange(anIndex, 1)];
 }
 
diff --git a/Tests/Foundation/CPIndexSetTest.j b/Tests/Foundation/CPIndexSetTest.j
index fefbd220d..adc0a0f43 100644
--- a/Tests/Foundation/CPIndexSetTest.j
+++ b/Tests/Foundation/CPIndexSetTest.j
@@ -202,6 +202,7 @@ function descriptionWithoutEntity(aString)
     [self assertTrue:[[CPIndexSet indexSetWithIndex:1] containsIndex:1]];
     [self assertTrue:[[CPIndexSet indexSetWithIndex:0] containsIndex:0]];
     [self assertFalse:[[CPIndexSet indexSetWithIndex:0] containsIndex:1]];
+    [self assertThrows:function() { [CPIndexSet indexSetWithIndex:NaN] }];
 }
 
 - (void)testIndexSetWithIndexesInRange