Found:

accidental gap

MORPHOLOGY: a non-existing word which is expected to exist given the hypothesized morphological rules of a particular language. In the literature, an accidental gap is usually thought of as a 'hole' in a paradigm. EXAMPLE: in English it is possible to derive nouns from verbs by adding the suffixes -al and -(a)tion to the verbal stem. However, some such derivations do not exist, although there are no grammatical reasons for their nonexistence. Compare the following examples:

(i)   recite	  recital      recitation
      propose	  proposal     proposition
(ii)  arrive	  arrival     *arrivation
      refuse	  refusal     *refusation
(iii) derive	 *derival      derivation
      describe   *describal    description