diff --git a/music_publisher/models.py b/music_publisher/models.py
index b45dab1..03213e3 100644
--- a/music_publisher/models.py
+++ b/music_publisher/models.py
@@ -1471,7 +1471,7 @@ def version(self):
         """Return CWR version."""
         if self.nwr_rev in ["WRK", "ISR"]:
             return "30"
-        elif self.nwr_rev == ["WR1", "IS1"]:
+        elif self.nwr_rev in ["WR1", "IS1"]:
             return "31"
         elif self.nwr_rev in ["NW2", "RE2"]:
             return "22"