diff --git a/src/cert.rs b/src/cert.rs
index 50ddd5f..44429d7 100644
--- a/src/cert.rs
+++ b/src/cert.rs
@@ -75,8 +75,8 @@ fn build_certificate(domain: &str) -> Result<Certificate, String> {
 
 fn validity_period() -> Result<(OffsetDateTime, OffsetDateTime), String> {
     let start = OffsetDateTime::now_utc().checked_sub(Duration::hours(1))
-        .ok_or::<String>("Generate start datetime failed.".into())?;
+        .ok_or_else::<String, _>(|| "Generate start datetime failed.".into())?;
     let end = OffsetDateTime::now_utc().checked_add(Duration::days(90))
-        .ok_or::<String>("Generate start datetime failed.".into())?;
+        .ok_or_else::<String, _>(|| "Generate start datetime failed.".into())?;
     Ok((start, end))
 }