diff --git a/kitty/key_encoding.c b/kitty/key_encoding.c
index a5059855a..04f18435c 100644
--- a/kitty/key_encoding.c
+++ b/kitty/key_encoding.c
@@ -33,7 +33,16 @@ typedef struct {
 
 static inline bool
 is_modifier_key(uint32_t key) {
-    return GLFW_FKEY_LEFT_SHIFT <= key && key <= GLFW_FKEY_RIGHT_SUPER;
+    START_ALLOW_CASE_RANGE
+    switch (key) {
+        case GLFW_FKEY_LEFT_SHIFT ...  GLFW_FKEY_RIGHT_SUPER:
+        case GLFW_FKEY_CAPS_LOCK:
+        case GLFW_FKEY_SCROLL_LOCK:
+        case GLFW_FKEY_NUM_LOCK:
+            return true;
+        default:
+            return false;
+    }
 }
 
 static inline void