The type half of check_matrix(), split out because the shape rules differ
between entry points but this rule does not. On the pre-computed
(theta, x) path a row is one simulation, so a bare vector is a column of
values rather than check_matrix()'s single row; a character or factor
column is the same mistake in either place. Left to
storage.mode(x) <- "double" such a column becomes all NA, every row is
then dropped as non-finite, and the error blames a simulator that was never
called.