--- a/servo/components/style/properties/helpers/animated_properties.mako.rs
+++ b/servo/components/style/properties/helpers/animated_properties.mako.rs
@@ -848,16 +848,17 @@ impl <T> Animatable for Option<T>
     }
 
     #[inline]
     fn compute_squared_distance(&self, other: &Self) -> Result<f64, ()> {
         match (self, other) {
             (&Some(ref this), &Some(ref other)) => {
                 this.compute_squared_distance(other)
             },
+            (&None, &None) => Ok(0.0),
             _ => Err(()),
         }
     }
 }
 
 /// https://drafts.csswg.org/css-transitions/#animtype-number
 impl Animatable for f32 {
     #[inline]