diff --git a/contrib/pm/main.php b/contrib/pm/main.php
index ae75d228..0e050f55 100755
--- a/contrib/pm/main.php
+++ b/contrib/pm/main.php
@@ -67,6 +67,8 @@ class PrivMsg extends SimpleExtension {
 
 		if($config->get_int("pm_version") < 2) {
 			log_info("pm", "Adding foreign keys to private messages");
+			$database->Execute("delete from private_message where to_id not in (select id from users);");
+			$database->Execute("delete from private_message where from_id not in (select id from users);");
 			$database->Execute("ALTER TABLE private_message 
 			ADD CONSTRAINT foreign_private_message_from_id FOREIGN KEY (from_id) REFERENCES users(id) ON DELETE CASCADE,
 			ADD CONSTRAINT foreign_private_message_to_id FOREIGN KEY (to_id) REFERENCES users(id) ON DELETE CASCADE;");