diff --git a/main.go b/main.go
index d30a22f..c78d038 100644
--- a/main.go
+++ b/main.go
@@ -230,6 +230,9 @@ func init() {
 }
 
 func dumpFSM(mapper *mapper.MetricMapper, dumpFilename string, logger log.Logger) error {
+	if mapper.FSM == nil {
+		return fmt.Errorf("no FSM available to be dumped, possibly because the mapping contains regex patterns")
+	}
 	f, err := os.Create(dumpFilename)
 	if err != nil {
 		return err