How to call system having trap("CHLD", "IGNORE")?

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Hi all,

doing some forks I want to ignore SIGCHLD. But

trap("CHLD", "IGNORE")
system "echo"

ends in ruby 1.8.2 (2005-04-11) [i386-linux] with a following error:

in `system': no implicit conversion from nil to integer (TypeError)

Where I am wrong?

Thanks,

P.