diff --git a/lib/replace/Dimension.js b/lib/replace/Dimension.js
index edb00388..49ff7dae 100644
--- a/lib/replace/Dimension.js
+++ b/lib/replace/Dimension.js
@@ -50,7 +50,7 @@ export default function compressDimension(node, item) {
         }
 
         // issue #222: don't remove units inside calc
-        if (this.function && MATH_FUNCTIONS.has(this.function.name)) {
+        if (this.function && MATH_FUNCTIONS.has(this.function.name.toLowerCase())) {
             return;
         }