in lines https://github.com/tagucci/pythonrouge/blob/master/pythonrouge/pythonrouge.py#L159-L165, the for loop accesses references before individual summaries. You have: ``` for j, ref in enumerate(self.reference): for k, doc in enumerate(ref): ``` Since, ``` reference = [ [[summaryA_ref1_sent1, summaryA_ref1_sent2], [summaryA_ref2_sent1, summaryA_ref2_sent2]], [[summaryB_ref1_sent1, summaryB_ref1_sent2], [summaryB_ref2_sent1, summaryB_ref2_sent2]] ] ``` your for loop should access summaries first such that: ``` for k, doc in enumerate(self.reference): for j, ref in enumerate(doc): ```