diff --git a/dom0/qvm-tools/qubes-prefs b/dom0/qvm-tools/qubes-prefs
index f135d428..bc8ce214 100755
--- a/dom0/qvm-tools/qubes-prefs
+++ b/dom0/qvm-tools/qubes-prefs
@@ -119,7 +119,7 @@ def main():
         qvm_collection.unlock_db()
 
     if options.do_set:
-        if len (args) < 2:
+        if len (args) < 2 or args[0] not in preferences.keys():
             print >> sys.stderr, "You must specify the property and the new value you wish to set..."
             print >> sys.stderr, "Available properties:"
             for p in sorted(preferences.keys()):
@@ -133,7 +133,7 @@ def main():
         qvm_collection.unlock_db()
 
     elif options.do_get:
-        if len (args) < 1:
+        if len (args) < 1 or args[0] not in preferences.keys():
             print >> sys.stderr, "You must specify the property you wish to get..."
             print >> sys.stderr, "Available properties:"
             for p in sorted(preferences.keys()):