diff --git a/doc/man/cgrules.conf.5 b/doc/man/cgrules.conf.5

index 7a89fb5..dd222a9 100644

--- a/doc/man/cgrules.conf.5

+++ b/doc/man/cgrules.conf.5

@@ -50,6 +50,10 @@ can be:

 .nf

     - comma separated controller names (no spaces) or 

     - * (for all mounted controllers)

+

+    make sure to define only controllers that are configured

+    for the matching \fIdestination\fR in \fBcgconfig.conf\fR,

+    otherwise the rule can fail

 .fi

 .I destination