diff --git a/focus_sequence_FLI.py b/focus_sequence_FLI.py
index f995430..db2acef 100755
--- a/focus_sequence_FLI.py
+++ b/focus_sequence_FLI.py
@@ -57,7 +57,7 @@ if __name__ == "__main__":
     parser.add_argument("focus_start", help="focus start value", type=float)
     parser.add_argument("focus_stop", help="focus stop value", type=float)
     parser.add_argument(
-        "focus_step", help="focus step value", type=float, nargs="*", default=np.nan
+        "focus_step", help="focus step value", type=float, nargs="?", default=np.nan
     )
 
     parser.add_argument(