diff --git a/phpBB/phpbb/notification/type/bookmark.php b/phpBB/phpbb/notification/type/bookmark.php
index 5e6fdd2523..003998677d 100644
--- a/phpBB/phpbb/notification/type/bookmark.php
+++ b/phpBB/phpbb/notification/type/bookmark.php
@@ -75,7 +75,7 @@ class bookmark extends \phpbb\notification\type\post
 		$result = $this->db->sql_query($sql);
 		while ($row = $this->db->sql_fetchrow($result))
 		{
-			$users[] = $row['user_id'];
+			$users[] = (int) $row['user_id'];
 		}
 		$this->db->sql_freeresult($result);
 
diff --git a/phpBB/phpbb/notification/type/post.php b/phpBB/phpbb/notification/type/post.php
index bc42c4422b..b2ad8ff33d 100644
--- a/phpBB/phpbb/notification/type/post.php
+++ b/phpBB/phpbb/notification/type/post.php
@@ -103,7 +103,7 @@ class post extends \phpbb\notification\type\base
 		$result = $this->db->sql_query($sql);
 		while ($row = $this->db->sql_fetchrow($result))
 		{
-			$users[] = $row['user_id'];
+			$users[] = (int) $row['user_id'];
 		}
 		$this->db->sql_freeresult($result);
 
@@ -115,7 +115,7 @@ class post extends \phpbb\notification\type\base
 		$result = $this->db->sql_query($sql);
 		while ($row = $this->db->sql_fetchrow($result))
 		{
-			$users[] = $row['user_id'];
+			$users[] = (int) $row['user_id'];
 		}
 		$this->db->sql_freeresult($result);
 
diff --git a/phpBB/phpbb/notification/type/quote.php b/phpBB/phpbb/notification/type/quote.php
index e8527261d8..745430e114 100644
--- a/phpBB/phpbb/notification/type/quote.php
+++ b/phpBB/phpbb/notification/type/quote.php
@@ -94,7 +94,7 @@ class quote extends \phpbb\notification\type\post
 		$result = $this->db->sql_query($sql);
 		while ($row = $this->db->sql_fetchrow($result))
 		{
-			$users[] = $row['user_id'];
+			$users[] = (int) $row['user_id'];
 		}
 		$this->db->sql_freeresult($result);
 
diff --git a/phpBB/phpbb/notification/type/topic.php b/phpBB/phpbb/notification/type/topic.php
index 98f086a50b..635d05bccd 100644
--- a/phpBB/phpbb/notification/type/topic.php
+++ b/phpBB/phpbb/notification/type/topic.php
@@ -103,7 +103,7 @@ class topic extends \phpbb\notification\type\base
 		$result = $this->db->sql_query($sql);
 		while ($row = $this->db->sql_fetchrow($result))
 		{
-			$users[] = $row['user_id'];
+			$users[] = (int) $row['user_id'];
 		}
 		$this->db->sql_freeresult($result);