diff --git a/src/string.ts b/src/string.ts
index 40de090d..6ff0a1a0 100644
--- a/src/string.ts
+++ b/src/string.ts
@@ -4,7 +4,7 @@
  * capitalize('hello')   -> 'Hello'
  * capitalize('va va voom') -> 'Va va voom'
  */
-export const capitalize = (str: string): string => {
+export const capitalize = <const Str extends string>(str: Str): Capitalize<Str> => {
   if (!str || str.length === 0) return ''
   const lower = str.toLowerCase()
   return lower.substring(0, 1).toUpperCase() + lower.substring(1, lower.length)