diff --git a/src/util/extend.ts b/src/util/extend.ts
index 1618656..ce86873 100644
--- a/src/util/extend.ts
+++ b/src/util/extend.ts
@@ -25,7 +25,7 @@ export type Extend<T extends readonly object[]> = (
     T extends [
         infer Super,
         infer Self,
-        ...infer Rest extends object[],
+        ...infer Rest extends readonly object[],
     ]
         ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
             ? Extend<[
@@ -42,7 +42,7 @@ export type Extendable<T extends readonly object[]> = (
     T extends [
         infer Super,
         infer Self,
-        ...infer Rest extends object[],
+        ...infer Rest extends readonly object[],
     ]
         ? Pick<Self, CommonKeys<Self, Super>> extends Pick<Super, CommonKeys<Self, Super>>
             ? Extendable<[
@@ -57,7 +57,7 @@ export type NonExtendableKeys<T extends readonly object[]> = (
     T extends [
         infer Super   extends object,
         infer Self    extends object,
-        ...infer Rest extends object[],
+        ...infer Rest extends readonly object[],
     ]
         ? {[K in keyof Super & keyof Self]: Self[K] extends Super[K]
             ? never