diff --git a/dix/devices.c b/dix/devices.c
index be236dd70..5f98afc42 100644
--- a/dix/devices.c
+++ b/dix/devices.c
@@ -2766,9 +2766,10 @@ AllocDevicePair(ClientPtr client, const char *name,
     keyboard->type = (master) ? MASTER_KEYBOARD : SLAVE;
 
     /* The ClassesRec stores the device classes currently not used. */
-    pointer->unused_classes = calloc(1, sizeof(ClassesRec));
-
-    keyboard->unused_classes = calloc(1, sizeof(ClassesRec));
+    if (IsMaster(pointer)) {
+        pointer->unused_classes = calloc(1, sizeof(ClassesRec));
+        keyboard->unused_classes = calloc(1, sizeof(ClassesRec));
+    }
 
     *ptr = pointer;