We present the first large scale quantitative investigation of the syncretisation of verbal subject agreement in Medieval French and test a classic analysis which relates non-syncretic agreement and null subjects as parts of the same grammar (e.g. Rizzi 1986, Adams 1987, Alexiadou & Anagnostopoulou 1998, Roberts 2010, Sheehan to appear). We show that agreement syncretisation and the emergence of overt pronominal subjects proceeded at the same rate. On the Constant Rate Hypothesis of Kroch (1989), which states that a grammatical change has the same rate in different contexts, these results are compatible with the traditional analysis. However, we show that this analysis also generates a number of predictions which are not borne out by the quantitative data. We conclude that a more complex model of interaction of subject and inflection parameters is needed.