diff --git a/Sources/JWT/Application+JWT.swift b/Sources/JWT/Application+JWT.swift
index 15a41e4..76062f9 100644
--- a/Sources/JWT/Application+JWT.swift
+++ b/Sources/JWT/Application+JWT.swift
@@ -42,7 +42,7 @@ extension Application {
 
         public var keys: JWTKeyCollection {
             get { self.storage.keys }
-            set { self.storage.keys = newValue }
+            nonmutating set { self.storage.keys = newValue }
         }
 
         private var storage: Storage {