Unable to C-a in -nographic if -serial telnet

qemu-system-i386 (version 2.6.1, running on Linux/x86_64) started with:

qemu-system-i386 -m 64M -machine type=pc -rtc base=localtime,clock=host -nographic -serial telnet:127.0.0.1:1234,server,nowait -net nic,model=ne2k_pci -net user,hostfwd=tcp:127.0.0.1:2200-:22,tftp=/

does not accept the escape key (C-a) to perform functions such as switching from monitor to console. Verified both in GNU screen and in the Linux console.

If '-serial telnet:127.0.0.1:1234,server,nowait' is removed from the command line, the escape key is accepted (and Qemu doesn't enter the monitor immediately).