diff --git a/acme/crypto.go b/acme/crypto.go
index 8be4c76a..9e8f3f0b 100644
--- a/acme/crypto.go
+++ b/acme/crypto.go
@@ -60,8 +60,7 @@ func generateDerCert(privKey *rsa.PrivateKey, expiration time.Time, domain strin
 		return nil, err
 	}
 
-	zero := time.Time{}
-	if expiration == zero {
+	if expiration.IsZero() {
 		expiration = time.Now().Add(365)
 	}