diff --git a/workflow/scripts/experiment_qc.py b/workflow/scripts/experiment_qc.py
index a5f1c81813d845512c44fc5c3f8eee91183f180c..7386fcffd8e982d4d432e99becfc2aacb23ad2f2 100644
--- a/workflow/scripts/experiment_qc.py
+++ b/workflow/scripts/experiment_qc.py
@@ -36,6 +36,7 @@ def get_args():
 
     parser.add_argument('-e', '--extension',
                         help="Number of base pairs to extend the reads",
+                        type=int,
                         required=True)
 
     args = parser.parse_args()