diff --git a/moto/rds/models.py b/moto/rds/models.py
index 289ec9906..3ce005e5e 100644
--- a/moto/rds/models.py
+++ b/moto/rds/models.py
@@ -37,7 +37,6 @@ class Database(object):
         self.source_db_identifier = kwargs.get("source_db_identifier")
         self.db_instance_class = kwargs.get('db_instance_class')
         self.port = kwargs.get('port')
-        self.db_instance_identifier = kwargs.get('db_instance_identifier')
         self.db_name = kwargs.get("db_name")
         self.publicly_accessible = kwargs.get("publicly_accessible")
         if self.publicly_accessible is None:
@@ -65,6 +64,10 @@ class Database(object):
         # DBParameterGroupName
         # VpcSecurityGroupIds.member.N
 
+    @property
+    def physical_resource_id(self):
+        return self.db_instance_identifier
+
     @property
     def address(self):
         return "{0}.aaaaaaaaaa.{1}.rds.amazonaws.com".format(self.db_instance_identifier, self.region)
@@ -130,7 +133,7 @@ class Database(object):
         source_db_identifier = properties.get("SourceDBInstanceIdentifier")
         if source_db_identifier:
             # Replica
-            db_kwargs["source_db_identifier"] = source_db_identifier.db_instance_identifier
+            db_kwargs["source_db_identifier"] = source_db_identifier
             database = rds_backend.create_database_replica(db_kwargs)
         else:
             database = rds_backend.create_database(db_kwargs)