If I remember right the error is seen when the script tries executing nrstop as user root.
I thought that this had been fixed in the later Signature Updates, but the bug may have sneaked back into the code.
When I did see this in the past, the error could generally be ignored. As long as the daemons are properly restarted, and the new signatures are added to packetd.conf then you are probably fine.
If you are concerned, then you may want to do an nrtsop and nrstart as netrangr after the install to ensure that the daemons did get restarted properly after the installation of the signature update.
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