diff --git a/sky/sdk/lib/base/lerp.dart b/sky/sdk/lib/base/lerp.dart
index db4b8d9d78c6b8266ada0fd0a5e29a01d34ae1be..b2bbbc974f1bcc051b28d1c36752a957a8bfe561 100644
--- a/sky/sdk/lib/base/lerp.dart
+++ b/sky/sdk/lib/base/lerp.dart
@@ -22,7 +22,7 @@ Color lerpColor(Color a, Color b, double t) {
   if (a == null && b == null)
     return null;
   if (a == null)
-    return _scaleAlpha(b, t);
+    return _scaleAlpha(a, t);
   if (b == null)
     return _scaleAlpha(b, 1.0 - t);
   return new Color.fromARGB(