diff --git a/types/styled-system/dist/util.d.ts b/types/styled-system/dist/util.d.ts
index f91b2b6620..ae90de60f8 100644
--- a/types/styled-system/dist/util.d.ts
+++ b/types/styled-system/dist/util.d.ts
@@ -19,7 +19,7 @@ export interface LowLevelStylefunctionArguments {
     cssProperty?: string;
     key?: string;
     getter?: () => any;
-    transformValue?: () => any;
+    transformValue?: (n: string|number) => any;
     scale?: Array<string|number>;
 }