diff --git a/AppKit/CPScroller.j b/AppKit/CPScroller.j
index 4984a4f16..c32e720f3 100644
--- a/AppKit/CPScroller.j
+++ b/AppKit/CPScroller.j
@@ -259,7 +259,7 @@ CPThemeStateScrollerKnobDark    = CPThemeState("scroller-knob-dark");
 - (void)setKnobProportion:(float)aProportion
 {
     if (!_IS_NUMERIC(aProportion))
-        [CPException raise:CPInvalidArgumentException reason:"aProportion must be numeric"];
+        [CPException raise:CPInvalidArgumentException reason:"aProportion must be numeric, was: "+aProportion];
 
     _knobProportion = MIN(1.0, MAX(0.0001, aProportion));