NMatrix#det_exact not working on :object dtype

[translunar]

t = N[[2,2],[3,4], dtype: :object]
t.det # Gives NotImplementedError, more or less as expected
t.det_exact
=> 
Cannot create ruby object
DataTypeError: Conversion to RubyObject requested from unknown/invalid data type (did you try to convert from a VALUE?)

I believe the problem is somewhere around

nmatrix/ext/nmatrix/math.cpp

Line 197 in 0de5c43

void det_exact(const int M, const void* A_elements, const int lda, void* result_arg) {

I recall seeing this type of error when an operator wasn't properly implemented on the RubyObject class, but I could be mistaken about that.

This issue blocks symengine/symengine.rb#36.

[translunar]

@wlevine, does this look familiar to you at all?