diff --git a/game/campaignloader/campaignairwingconfig.py b/game/campaignloader/campaignairwingconfig.py
index 6146624e..9bac5257 100644
--- a/game/campaignloader/campaignairwingconfig.py
+++ b/game/campaignloader/campaignairwingconfig.py
@@ -39,11 +39,18 @@ class SquadronConfig:
         else:
             secondary = [FlightType(s) for s in secondary_raw]
 
+        max_size = data.get("size", DEFAULT_SQUADRON_SIZE)
+        if max_size is None:
+            raise RuntimeError(
+                "Squadron's size is defined in campaign but is missing a value:\n"
+                f"{data}"
+            )
+
         return SquadronConfig(
             FlightType(data["primary"]),
             secondary,
             data.get("aircraft", []),
-            data.get("size", DEFAULT_SQUADRON_SIZE),
+            max_size,
             data.get("name", None),
             data.get("nickname", None),
             data.get("female_pilot_percentage", None),