Unclear error message if matrix is used

[FlorianSeitz]

cognitivemodels/tests/testthat/test_ebmj.R

Line 205 in dd3ab97

ebm_j(rp ~ f1 + f2, ~c, data = as.matrix(dt), fix = pars)

I get the following error:
Error in private$get_input(f = self$formulaCriterion, d = d, na.action = NULL) :
Can't find variables from 'formula' in 'data': [c].

This error seems a bit counterintuitive as we only use a matrix rather than a data frame.