Which contrast correctly limits the adjective rule?

Cause of Ghayr Munsarif: Additional Alif and Nun

• Both qualify merely because they end in ان
• Neither has a feminine
• Both are proper names
• سَكْرَانُ → سَكْرَىٰ qualifies; نَدْمَانٌ → نَدْمَانَةٌ does not qualify through this test