diff --git a/main.go b/main.go
index e658416..3d95b07 100644
--- a/main.go
+++ b/main.go
@@ -100,6 +100,8 @@ func main() {
 
 	if *percent < 0 || *percent > 1 {
 		log.Fatal("invalid percentage")
+	} else if *nevents < 0 {
+		log.Fatal("number of events must be non-negative")
 	}
 
 	t := time.Now().Unix() - formula(*percent)