diff --git a/src/array/initial.ts b/src/array/initial.ts
index 291b6e80..2870933d 100644
--- a/src/array/initial.ts
+++ b/src/array/initial.ts
@@ -21,7 +21,7 @@
  * const newSingleElementArr = initial(singleElementArr);
  * // newSingleElementArr will be []
  */
-export function initial<T>(arr: T[]): T[] {
+export function initial<T>(arr: readonly T[]): T[] {
   if (arr.length <= 1) {
     return [];
   }