AddComment method

<?xml version='1.0' encoding='UTF-8'?>

<methodResponse>

<fault>

<value>

<struct>

<member>

<name>faultString</name>

<value>

<string>Client error: Not allowed to create comment</string>

</value>

</member>

<member>

<name>faultCode</name>

<value>

<int>337</int>

</value>

</member>

</struct>

</value>

</fault>

</methodResponse>

I see this version of source code (http://repo.or.cz/w/livejournal.git/blob/HEAD:/cgi-bin/ljprotocol.pl#l560) I see that this error code returns if LJ::Comment->create failed but I don't understand why does it fail?
For my posts I can add comments without problem.