diff --git a/tools/python/xen/xm/main.py b/tools/python/xen/xm/main.py
index ebb7e644e5..9e4a4a30dc 100644
--- a/tools/python/xen/xm/main.py
+++ b/tools/python/xen/xm/main.py
@@ -460,6 +460,8 @@ def xm_mem_set(args):
     server.xend_domain_mem_target_set(dom, mem_target)
     
 def xm_set_vcpus(args):
+    arg_check(args, 2, "set-vcpus")
+    
     from xen.xend.XendClient import server
     server.xend_domain_set_vcpus(args[0], int(args[1]))