diff --git a/arn/validate/Validate.go b/arn/validate/Validate.go
index 5d38d745..ffc633c4 100644
--- a/arn/validate/Validate.go
+++ b/arn/validate/Validate.go
@@ -60,7 +60,7 @@ func Date(date string) bool {
 
 // YearMonth tells you whether the date contain only the year and the month.
 func YearMonth(date string) bool {
-	if date == "" || strings.HasPrefix(date, "0001") {
+	if len(date) != len(YearMonthFormat) || strings.HasPrefix(date, "0001") {
 		return false
 	}