diff --git a/mddatasetbuilder/qmcalc.py b/mddatasetbuilder/qmcalc.py
index a5eaa8e..ab9c540 100644
--- a/mddatasetbuilder/qmcalc.py
+++ b/mddatasetbuilder/qmcalc.py
@@ -30,6 +30,7 @@ def _commandline():
         "--ncpus",
         help="Number of CPU cores used, if not set, all CPU cores are used",
         default=None,
+        type=int,
     )
     args = parser.parse_args()
     qmcalc(gjfdir=args.dir, command=args.command)