diff --git a/lib/aos/aos_urpc.c b/lib/aos/aos_urpc.c
index 8af34df..3f4827d 100644
--- a/lib/aos/aos_urpc.c
+++ b/lib/aos/aos_urpc.c
@@ -68,10 +68,14 @@ errval_t do_aos_urpc(
     if (ret0 != NULL) *ret0 = rpc->meta->a2;
     if (ret1 != NULL) *ret1 = rpc->meta->a3;
 
-    if(ret_cap != NULL && rpc->meta->cap.type != ObjType_Null) {
-        assert(rpc->meta->cap.type == ObjType_RAM);
-        err = ram_forge(*ret_cap, rpc->meta->cap.u.ram.base, rpc->meta->cap.u.ram.bytes, my_core_id);
-        if (err_is_fail(err)) return err;
+    if(ret_cap != NULL) {
+        if(rpc->meta->cap.type != ObjType_Null) {
+            assert(rpc->meta->cap.type == ObjType_RAM);
+            err = ram_forge(*ret_cap, rpc->meta->cap.u.ram.base, rpc->meta->cap.u.ram.bytes, my_core_id);
+            if (err_is_fail(err)) return err;
+        } else {
+            *ret_cap = NULL_CAP;
+        }
     }
 
     return rpc->meta->a0;