多因素方差分析ols报错（已解决）

做多因素方差分析 运行ols报错，请大神指点如何修改df=creditcard.dropna()
from statsmodels.formula.api import ols
from statsmodels.stats.anova import anova_lm
ols_result=ols('avg_exp ~ C(edu_class)',data=df).fit()
报错如下：
Traceback (most recent call last):
  File "<ipython-input-44-b34699b68092>", line 1, in <module>
    ols_result=ols('avg_exp ~ C(edu_class)',data=df).fit()

  File "D:\anaconda\lib\site-packages\statsmodels\base\model.py", line 155, in from_formula
    missing=missing)

  File "D:\anaconda\lib\site-packages\statsmodels\formula\formulatools.py", line 65, in handle_formula_data
    NA_action=na_action)

  File "D:\anaconda\lib\site-packages\patsy\highlevel.py", line 310, in dmatrices
    NA_action, return_type)

  File "D:\anaconda\lib\site-packages\patsy\highlevel.py", line 165, in _do_highlevel_design
    NA_action)

  File "D:\anaconda\lib\site-packages\patsy\highlevel.py", line 62, in _try_incr_builders
    formula_like = ModelDesc.from_formula(formula_like)

  File "D:\anaconda\lib\site-packages\patsy\desc.py", line 164, in from_formula
    tree = parse_formula(tree_or_string)

  File "D:\anaconda\lib\site-packages\patsy\parse_formula.py", line 148, in parse_formula
    _atomic_token_types)

  File "D:\anaconda\lib\site-packages\patsy\infix_parser.py", line 210, in infix_parse
    for token in token_source:

  File "D:\anaconda\lib\site-packages\patsy\parse_formula.py", line 94, in _tokenize_formula
    yield _read_python_expr(it, end_tokens)

  File "D:\anaconda\lib\site-packages\patsy\parse_formula.py", line 44, in _read_python_expr
    for pytype, token_string, origin in it:

  File "D:\anaconda\lib\site-packages\patsy\util.py", line 332, in next
    return six.advance_iterator(self._it)

  File "D:\anaconda\lib\site-packages\patsy\tokens.py", line 35, in python_tokenize
    assert pytype not in (tokenize.NL, tokenize.NEWLINE)

AssertionError