diff --git a/openwisp_radius/api/serializers.py b/openwisp_radius/api/serializers.py
index 0245d5ab..6eeadcf4 100644
--- a/openwisp_radius/api/serializers.py
+++ b/openwisp_radius/api/serializers.py
@@ -120,7 +120,7 @@ class RadiusPostAuthSerializer(serializers.ModelSerializer):
         allow_blank=True,
         style={'input_type': 'password'},
     )
-    called_station_id = serializers.CharField(required=False, allow_blank=True)
+    called_station_id = serializers.CharField(required=False, allow_blank=True, max_length=50)
     calling_station_id = serializers.CharField(required=False, allow_blank=True)
 
     def validate(self, data):