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