diff --git a/sys/amd64/vmm/vmm_ioport.c b/sys/amd64/vmm/vmm_ioport.c
index e494bac46f2..e30f796d598 100644
--- a/sys/amd64/vmm/vmm_ioport.c
+++ b/sys/amd64/vmm/vmm_ioport.c
@@ -157,7 +157,7 @@ emulate_inout_str(struct vm *vm, int vcpuid, struct vm_exit *vmexit, bool *retu)
 int
 vm_handle_inout(struct vm *vm, int vcpuid, struct vm_exit *vmexit, bool *retu)
 {
-	int bytes, error;
+	int bytes __diagused, error;
 
 	bytes = vmexit->u.inout.bytes;
 	KASSERT(bytes == 1 || bytes == 2 || bytes == 4,