The aim of this paper is to analyze the affix ‑m(i)entre of modal adverbs in Medieval Spanish, e.g. fuertemientre. The hypothesis is that the non-etymological, epenthetic r may be analyzed from a phonological perspective only, without grammatical or lexical explanations. The paper first reviews the three traditional explanations for the non-etymological r in modal adverbs, especially the analogical one, which poses that the r of these adverbs is an analogy from the etymological r of the temporal adverb m(i)entre. The paper shows the weakness of these traditional analyses. The paper then presents an alternative analysis on phonological basis, which has greater parsimony than traditional explanations, adding typological evidence outside the grammatical field of modal adverbs, and showing that a non-etymological r appeared in many Spanish words, and in other Romance languages, in the same phonetic and syllabic contexts as the modal affix ‑m(i)entre. Finally, the paper proposes three causes for the loss of modal ‑m(i)entre: the weakening of intra-paradigmatic support from modal ‑miente, a homonymic clash between modal ‑m(i)entre and temporal m(i)entre, and the generalization of the learned form ‑mente. The paper places the diachronic loss of the modal affix with r in a theoretical frame of multicausality.