It also isn't that strange for a language to have an affix that can't
attach to words with certain phonological shapes, even if the resulting
word wouldn't violate any phonological rules. For instance, the English
deadjectival verbalizing suffix "-en" that we see in words such as
"redden" and "darken" (it's generally used with color terms, but can
occasionally occur with other adjectives) cannot be added to adjectives
ending in a vowel or sonorant — which is why you don't hear *yellowen or
*greenen.