Why does this non-coinbase transaction not have a sender address? This is the transaction in concern: https://blockchain.info/rawtx/6b1c57d98a557b5f578c02f6c386ef049df5890ebddd01d286bf345f47c5cecc

I think it is a transaction having nothing to do with coinbase because there is a prev_out field in the items in the input array. However, there is no field "addr" in the prev_out item, unlike this transaciton: https://blockchain.info/rawtx/304b90ea66d80652f2473c0b455bfa7775fa32b2de3f170c15bfe8e65c0a7adc 
and most of the other transactions.

Is there any reason for lacking an "addr" field? Could this indicate some malpractice?
the transaction 6b1c57d98a557b5f578c02f6c386ef049df5890ebddd01d286bf345f47c5cecc spends multisignature output. 

blockchain.info does not resolve such kind of inputs to addresses