diff --git a/google/resource_kms_crypto_key.go b/google/resource_kms_crypto_key.go
index 95727c21..2a4a852c 100644
--- a/google/resource_kms_crypto_key.go
+++ b/google/resource_kms_crypto_key.go
@@ -38,7 +38,7 @@ func resourceKmsCryptoKey() *schema.Resource {
 			"rotation_period": {
 				Type:         schema.TypeString,
 				Optional:     true,
-				ValidateFunc: validateKmsCryptoKeyRotationPeriod,
+				ValidateFunc: orEmpty(validateKmsCryptoKeyRotationPeriod),
 			},
 			"version_template": {
 				Type:     schema.TypeList,