How to receive multicast data on a multihomed server's non-default interface

I have a linux server with two NICs (eth0 and eth1), and have set eth0 as default in "ip route." Now I would like to receive multicast packets on eth1. I have added "224.0.20.0/24 dev eth1 proto static scope link" to the routing table, and I connect as follows:

sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);

// port 12345, adress INADDR_ANY
bind(sock, &bind_addr, sizeof(bind_addr));

// multicast address 224.0.20.100, interface address 10.13.0.7 (=eth1)
setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &imreq, sizeof(imreq));

According to ip maddr it connects to that group on the right interface, and tshark -i eth1 shows that I am actually getting multicast packets.

However, I don't get any packets when calling recvfrom(sock). If I set "ip route default" to eth1 (instead of eth0), I do get packets via recvfrom. Is this an issue with my code or with my network setup, and what is the correct way of doing this?

(update) solution: caf hinted that this might be the same problem; indeed: after doing echo 0 > /proc/sys/net/ipv4/conf/eth1/rp_filter I can now receive multicast packets!