diff --git a/mhcflurry/multiallelic_refinement_command.py b/mhcflurry/multiallelic_refinement_command.py
index 63371e58d18929cd768c4595b1dbab5d24c5ac90..cc8ca67585eae88a10d93d4884675934057bc304 100644
--- a/mhcflurry/multiallelic_refinement_command.py
+++ b/mhcflurry/multiallelic_refinement_command.py
@@ -74,8 +74,9 @@ parser.add_argument(
     default=None)
 parser.add_argument(
     "--only-alleles-with-mass-spec",
-    type=int,
-    default=None)
+    type=bool,
+    default=False,
+    action="store_true")
 parser.add_argument(
     "--verbosity",
     type=int,