Consider section 121a4, which lists a variety of consonant stem adjectives that do not take -i in the ablative singular. They are completely regular, and the entry is really there only to keep you from having second guesses.
I’ll list them here so they get some web mileage, despite not being especially interesting, however rare:
- caeles, caelitis relating to the heavens or their Gods
- compos, compotis possessing control of
- dēses, dēsidis lazy
- dīves, dīvitis wealthy
- hospes, hospitis amicable, relating to guest-friendship
- particeps, participis participating in
- praepes, praepitis nimble, winged
- pauper, pauperis poor, destitute
- prīnceps, prīncipis princely, noble
- sōspes, sospitis safe and sound
- superstes, superstitis surviving
As someone pointed out in a comment, the general but non-binding idea is that those adjectives which most often operate as nouns (like these) take the ablative in -e, whereas those that are properly adjectival take the ablative in -i. You find hints of this throughout the consonantal and i-stem entries in Allen and Greenough (see 121a1-2), but they make no effort to propagate it as a formal rule.
The Essential AG: 121a1-2, a4
Latin for Addicts 