diff --git a/flopy/mf6/utils/mfsimlistfile.py b/flopy/mf6/utils/mfsimlistfile.py
index c7270711b3..9e5c0e53e5 100644
--- a/flopy/mf6/utils/mfsimlistfile.py
+++ b/flopy/mf6/utils/mfsimlistfile.py
@@ -83,7 +83,7 @@ def get_runtime(
         if units not in UNITS:
             msg = (
                 "units input variable must be "
-                + " ,".join(UNITS)
+                + ", ".join(UNITS)
                 + f": {units} was specified."
             )
             raise ValueError(msg)