diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 0c71b82c841..b8c2b8860d1 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -3723,14 +3723,14 @@ fn static_assert() {
                 }
 
                 if let Some(bottom) = rect.bottom {
-                    self.gecko.mClip.height = bottom.0 - self.gecko.mClip.y;
+                    self.gecko.mClip.height = (bottom - Au(self.gecko.mClip.y)).0;
                 } else {
                     self.gecko.mClip.height = 1 << 30; // NS_MAXSIZE
                     self.gecko.mClipFlags |= NS_STYLE_CLIP_BOTTOM_AUTO as u8;
                 }
 
                 if let Some(right) = rect.right {
-                    self.gecko.mClip.width = right.0 - self.gecko.mClip.x;
+                    self.gecko.mClip.width = (right - Au(self.gecko.mClip.x)).0;
                 } else {
                     self.gecko.mClip.width = 1 << 30; // NS_MAXSIZE
                     self.gecko.mClipFlags |= NS_STYLE_CLIP_RIGHT_AUTO as u8;