problem in df_lipinski = df['canonical_smiles'] code

Dear Sir, 
ArgumentError: Python argument types in
    rdkit.Chem.rdMolDescriptors._CalcMolWt(NoneType)
did not match C++ signature:
    _CalcMolWt(class RDKit::ROMol mol, bool onlyHeavy=False)
 this type error is showing in my notebook. Pl reply me to solve this.