diff --git a/src/sugar/graphics/icon.py b/src/sugar/graphics/icon.py
index d22b412c..24b5ca53 100644
--- a/src/sugar/graphics/icon.py
+++ b/src/sugar/graphics/icon.py
@@ -580,8 +580,8 @@ class CanvasIcon(hippo.CanvasBox, hippo.CanvasItem):
     
     palette = property(get_palette, set_palette)
 
-def get_icon_state(base_name, perc):
-    step = 5
+def get_icon_state(base_name, perc, step=5):
+    assert step > 0, 'get_icon_state(): step must be positive\n'
     strength = round(perc / step) * step
     icon_theme = gtk.icon_theme_get_default()