diff --git a/src/wp-includes/class-wp-object-cache.php b/src/wp-includes/class-wp-object-cache.php
index b0c1cc36f5..c22fd6298d 100644
--- a/src/wp-includes/class-wp-object-cache.php
+++ b/src/wp-includes/class-wp-object-cache.php
@@ -129,6 +129,43 @@ class WP_Object_Cache {
 		unset( $this->$name );
 	}
 
+	/**
+	 * Serves as a utility function to determine whether a key is valid.
+	 *
+	 * @since 6.1.0
+	 *
+	 * @param int|string $key Cache key to check for validity.
+	 * @return bool Whether the key is valid.
+	 */
+	protected function is_valid_key( $key ) {
+		if ( is_int( $key ) ) {
+			return true;
+		}
+
+		if ( is_string( $key ) && trim( $key ) !== '' ) {
+			return true;
+		}
+
+		$type = gettype( $key );
+
+		if ( ! function_exists( '__' ) ) {
+			wp_load_translations_early();
+		}
+
+		$message = is_string( $key )
+			? __( 'Cache key must not be an empty string.' )
+			/* translators: %s: The type of the given cache key. */
+			: sprintf( __( 'Cache key must be integer or non-empty string, %s given.' ), $type );
+
+		_doing_it_wrong(
+			sprintf( '%s::%s', __CLASS__, debug_backtrace( DEBUG_BACKTRACE_IGNORE_ARGS, 2 )[1]['function'] ),
+			$message,
+			'6.1.0'
+		);
+
+		return false;
+	}
+
 	/**
 	 * Serves as a utility function to determine whether a key exists in the cache.
 	 *
@@ -163,6 +200,10 @@ class WP_Object_Cache {
 			return false;
 		}
 
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
@@ -216,6 +257,10 @@ class WP_Object_Cache {
 	 * @return bool True if contents were replaced, false if original value does not exist.
 	 */
 	public function replace( $key, $data, $group = 'default', $expire = 0 ) {
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
@@ -245,14 +290,19 @@ class WP_Object_Cache {
 	 * more for cache plugins which use files.
 	 *
 	 * @since 2.0.0
+	 * @since 6.1.0 Returns false if cache key is invalid.
 	 *
 	 * @param int|string $key    What to call the contents in the cache.
 	 * @param mixed      $data   The contents to store in the cache.
 	 * @param string     $group  Optional. Where to group the cache contents. Default 'default'.
 	 * @param int        $expire Optional. Not used.
-	 * @return true Always returns true.
+	 * @return bool True if contents were set, false if key is invalid.
 	 */
 	public function set( $key, $data, $group = 'default', $expire = 0 ) {
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
@@ -310,6 +360,10 @@ class WP_Object_Cache {
 	 * @return mixed|false The cache contents on success, false on failure to retrieve contents.
 	 */
 	public function get( $key, $group = 'default', $force = false, &$found = null ) {
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
@@ -368,6 +422,10 @@ class WP_Object_Cache {
 	 * @return bool True on success, false if the contents were not deleted.
 	 */
 	public function delete( $key, $group = 'default', $deprecated = false ) {
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
@@ -416,6 +474,10 @@ class WP_Object_Cache {
 	 * @return int|false The item's new value on success, false on failure.
 	 */
 	public function incr( $key, $offset = 1, $group = 'default' ) {
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
@@ -455,6 +517,10 @@ class WP_Object_Cache {
 	 * @return int|false The item's new value on success, false on failure.
 	 */
 	public function decr( $key, $offset = 1, $group = 'default' ) {
+		if ( ! $this->is_valid_key( $key ) ) {
+			return false;
+		}
+
 		if ( empty( $group ) ) {
 			$group = 'default';
 		}
diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index 3c0f917bbc..416db2b86f 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -1850,8 +1850,11 @@ function update_user_caches( $user ) {
 
 	wp_cache_add( $user->ID, $user, 'users' );
 	wp_cache_add( $user->user_login, $user->ID, 'userlogins' );
-	wp_cache_add( $user->user_email, $user->ID, 'useremail' );
 	wp_cache_add( $user->user_nicename, $user->ID, 'userslugs' );
+
+	if ( ! empty( $user->user_email ) ) {
+		wp_cache_add( $user->user_email, $user->ID, 'useremail' );
+	}
 }
 
 /**
@@ -1878,9 +1881,12 @@ function clean_user_cache( $user ) {
 
 	wp_cache_delete( $user->ID, 'users' );
 	wp_cache_delete( $user->user_login, 'userlogins' );
-	wp_cache_delete( $user->user_email, 'useremail' );
 	wp_cache_delete( $user->user_nicename, 'userslugs' );
 
+	if ( ! empty( $user->user_email ) ) {
+		wp_cache_delete( $user->user_email, 'useremail' );
+	}
+
 	/**
 	 * Fires immediately after the given user's cache is cleaned.
 	 *
diff --git a/tests/phpunit/tests/user.php b/tests/phpunit/tests/user.php
index 8a3c59eeb4..60c388b71c 100644
--- a/tests/phpunit/tests/user.php
+++ b/tests/phpunit/tests/user.php
@@ -2010,6 +2010,7 @@ class Tests_User extends WP_UnitTestCase {
 		$update_data = array(
 			'ID'         => $create_user,
 			'user_login' => 'test_user',
+			'user_email' => 'user@example.com',
 			'meta_input' => array(
 				'test_meta_key' => 'test_meta_updated',
 				'custom_meta'   => 'updated_value',
diff --git a/tests/phpunit/tests/user/getUserCount.php b/tests/phpunit/tests/user/getUserCount.php
index 2ccfea78c9..f6a94d5624 100644
--- a/tests/phpunit/tests/user/getUserCount.php
+++ b/tests/phpunit/tests/user/getUserCount.php
@@ -45,7 +45,7 @@ class Tests_User_GetUserCount extends WP_UnitTestCase {
 		$current_network_user_count = get_user_count();
 
 		// Add another user to fake the network user count to be different.
-		wpmu_create_user( 'user', 'pass', 'email' );
+		wpmu_create_user( 'user', 'pass', 'user@example.com' );
 
 		wp_update_network_user_counts( $different_network_id );
 
@@ -68,7 +68,7 @@ class Tests_User_GetUserCount extends WP_UnitTestCase {
 		wp_update_network_counts();
 		$start_count = get_user_count();
 
-		wpmu_create_user( 'user', 'pass', 'email' );
+		wpmu_create_user( 'user', 'pass', 'user@example.com' );
 
 		// No change, cache not refreshed.
 		$count = get_user_count();
@@ -132,7 +132,7 @@ class Tests_User_GetUserCount extends WP_UnitTestCase {
 		wp_update_user_counts();
 		$current_network_user_count = get_user_count();
 
-		$u1 = wpmu_create_user( 'user', 'pass', 'email' );
+		$u1 = wpmu_create_user( 'user', 'pass', 'user@example.com' );
 
 		$user_count = get_user_count();
 
diff --git a/tests/phpunit/tests/user/slashes.php b/tests/phpunit/tests/user/slashes.php
index a45f7cfbf9..40eb2b176b 100644
--- a/tests/phpunit/tests/user/slashes.php
+++ b/tests/phpunit/tests/user/slashes.php
@@ -148,7 +148,7 @@ class Tests_User_Slashes extends WP_UnitTestCase {
 			array(
 				'user_login'   => 'slash_example_user_3',
 				'role'         => 'subscriber',
-				'email'        => 'user3@example.com',
+				'user_email'   => 'user3@example.com',
 				'first_name'   => self::SLASH_1,
 				'last_name'    => self::SLASH_3,
 				'nickname'     => self::SLASH_5,
@@ -169,7 +169,7 @@ class Tests_User_Slashes extends WP_UnitTestCase {
 			array(
 				'user_login'   => 'slash_example_user_4',
 				'role'         => 'subscriber',
-				'email'        => 'user3@example.com',
+				'user_email'   => 'user4@example.com',
 				'first_name'   => self::SLASH_2,
 				'last_name'    => self::SLASH_4,
 				'nickname'     => self::SLASH_6,