diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index 6a2bf20ca9..284b3d71b2 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -1191,6 +1191,7 @@ function wp_dropdown_users( $args = '' ) {
 		if ( $parsed_args['include_selected'] && ( $parsed_args['selected'] > 0 ) ) {
 			$found_selected          = false;
 			$parsed_args['selected'] = (int) $parsed_args['selected'];
+
 			foreach ( (array) $users as $user ) {
 				$user->ID = (int) $user->ID;
 				if ( $user->ID === $parsed_args['selected'] ) {
@@ -1199,7 +1200,10 @@ function wp_dropdown_users( $args = '' ) {
 			}
 
 			if ( ! $found_selected ) {
-				$users[] = get_userdata( $parsed_args['selected'] );
+				$selected_user = get_userdata( $parsed_args['selected'] );
+				if ( $selected_user ) {
+					$users[] = $selected_user;
+				}
 			}
 		}
 
diff --git a/tests/phpunit/tests/user/wpDropdownUsers.php b/tests/phpunit/tests/user/wpDropdownUsers.php
index 4f6b89db77..91c35cf63a 100644
--- a/tests/phpunit/tests/user/wpDropdownUsers.php
+++ b/tests/phpunit/tests/user/wpDropdownUsers.php
@@ -129,6 +129,22 @@ class Tests_User_WpDropdownUsers extends WP_UnitTestCase {
 		$this->assertContains( $user1->user_login, $found );
 	}
 
+	/**
+	 * @ticket 51370
+	 */
+	public function test_include_selected_with_non_existing_user_id() {
+		$found = wp_dropdown_users(
+			array(
+				'echo'             => false,
+				'selected'         => PHP_INT_MAX,
+				'include_selected' => true,
+				'show'             => 'user_login',
+			)
+		);
+
+		$this->assertNotContains( (string) PHP_INT_MAX, $found );
+	}
+
 	/**
 	 * @ticket 38135
 	 */