diff --git a/util/vm/build.py b/util/vm/build.py
index 9fb774f68c3e3edbffd09ca89222dbf2746287c9..a5f46b3e4963506a35cbba1b570ab862b52e5fa8 100755
--- a/util/vm/build.py
+++ b/util/vm/build.py
@@ -538,7 +538,7 @@ def interact( vm, tests, pre='', post='', prompt=Prompt ):
     log( '* Running VM install script' )
     installcmd = 'bash install-mininet-vm.sh'
     if Branch:
-        cmd = cmd + ' ' + Branch
+        installcmd += ' ' + Branch
     vm.sendline( installcmd )
     vm.expect ( 'password for mininet: ' )
     vm.sendline( 'mininet' )