problem with using my own corpus

hello everyone...
i have faced a problem since  3 days..When i import my own corpus i get this error:
raise ValueError("np.nan is an invalid document, expected byte or "
ValueError: np.nan is an invalid document, expected byte or unicode string.

i also checked  if there is any np.nan in my document but it seems there is not...Can u give me any hint why i am facing this error?

best regards