edit properties uses insert instead of update

Node creation without usage of protected node creates an entry in table protected_nodes where protected_node_is_protected = 0 and protected_node_password is empty.
Editing this node and changing to protected results in an SQL insert statement
INSERT INTO protected_nodes (protected_node_is_protected, protected_node_passwd, protected_node_show_title, nid) VALUES (1,
', , ) in protected_node.module on line 599
instead of an update statement