diff --git a/AK/FlyString.h b/AK/FlyString.h
index ea6411c26b..19ddd37f72 100644
--- a/AK/FlyString.h
+++ b/AK/FlyString.h
@@ -89,9 +89,8 @@ public:
 
     static void did_destroy_impl(Badge<StringImpl>, StringImpl&);
 
-    bool is_one_of() { return false; }
     template<typename T, typename... Rest>
-    bool is_one_of(const T& string, Rest... rest)
+    bool is_one_of(const T& string, Rest... rest) const
     {
         if (string == *this)
             return true;
@@ -100,6 +99,8 @@ public:
 
 
 private:
+    bool is_one_of() const { return false; }
+
     RefPtr<StringImpl> m_impl;
 };
 
diff --git a/AK/String.h b/AK/String.h
index 443a6b9549..6f99be9b5b 100644
--- a/AK/String.h
+++ b/AK/String.h
@@ -224,9 +224,8 @@ public:
 
     int replace(const String& needle, const String& replacement, bool all_occurences = false);
 
-    bool is_one_of() { return false; }
     template<typename T, typename... Rest>
-    bool is_one_of(const T& string, Rest... rest)
+    bool is_one_of(const T& string, Rest... rest) const
     {
         if (string == *this)
             return true;
@@ -234,6 +233,8 @@ public:
     }
 
 private:
+    bool is_one_of() const { return false; }
+
     RefPtr<StringImpl> m_impl;
 };