diff --git a/components/style/gecko/conversions.rs b/components/style/gecko/conversions.rs
index 2426bef42dc..8f3fb6bf2d0 100644
--- a/components/style/gecko/conversions.rs
+++ b/components/style/gecko/conversions.rs
@@ -102,6 +102,16 @@ impl From<nsStyleCoord_CalcValue> for LengthOrPercentage {
     }
 }
 
+impl From<nsStyleCoord_CalcValue> for LengthOrPercentageOrAuto {
+    fn from(other: nsStyleCoord_CalcValue) -> LengthOrPercentageOrAuto {
+        match (other.mHasPercent, other.mLength) {
+            (false, _) => LengthOrPercentageOrAuto::Length(Au(other.mLength)),
+            (true, 0) => LengthOrPercentageOrAuto::Percentage(Percentage(other.mPercent)),
+            _ => LengthOrPercentageOrAuto::Calc(other.into()),
+        }
+    }
+}
+
 impl From<Angle> for CoordDataValue {
     fn from(reference: Angle) -> Self {
         match reference {
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 3b92e207129..af74593c536 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -3434,7 +3434,7 @@ fn static_assert() {
                 LengthOrPercentageOrAuto::Auto
             } else {
                 debug_assert!(ty == DimensionType::eLengthPercentage as u8);
-                LengthOrPercentageOrAuto::Calc(value.into())
+                value.into()
             }
         }