diff --git a/lib/ui/lerp.dart b/lib/ui/lerp.dart
index c1b309da3ec6955efe1db54353b02c27c9b88797..d464dca2736e808de666ef81f8c8b53187695480 100644
--- a/lib/ui/lerp.dart
+++ b/lib/ui/lerp.dart
@@ -5,7 +5,7 @@
 part of dart_ui;
 
 /// Linearly interpolate between two numbers.
-num lerpDouble(num a, num b, double t) {
+double lerpDouble(num a, num b, double t) {
   if (a == null && b == null)
     return null;
   if (a == null)