Category slug field missing with registered custom taxonomy

I have registered a new taxonomy term using the same code on two different sites. On one site (a vanilla install), If I go to my taxonomy term in the admin menu, I see a ‘slug’ option for the add category screen. On my other site, with all plugins disabled, this slug option does not appear.

Anyone know what might cause this?

Register taxonomy code:

register_taxonomy( 'my_topics', 
array( 'post', 'page' ), 
array( 'hierarchical' => true, 
'label' => 'my Categories', 
'query_var' => 'topics', 
'rewrite' => array('slug' => 'topics')
) );