def METHOD_NAME(data):
    res = IVGMM(data.dep, data.exog, data.endog, data.instr).fit(cov_type="robust")
    match = "variables must be a str or a list of str"
    with pytest.raises(TypeError, match=match):
        res.c_stat(variables=1)
    with pytest.raises(TypeError, match=match):
        res.c_stat(variables=("x1", "x2"))