Can't enable IPv4 forwarding via adb shell

I've got a rooted Android phone and am trying to enable IPv4 forwarding via adb shell. If I do

host$ adb shell
phone$ su -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

then it works just fine. However,

host$ adb shell su -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

produces

/system/bin/sh: can't create /proc/sys/net/ipv4/ip_forward: Permission denied

even though adb shell cat /proc/sys/net/ipv4/ip_forward shows that the file already exists.