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