diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index dae7ed7e4c..7d1669caf1 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -966,23 +966,31 @@ function validate_theme_requirements( $stylesheet ) {
  * Retrieves all theme modifications.
  *
  * @since 3.1.0
+ * @since 5.9.0 The return value is always an array.
  *
- * @return array|void Theme modifications.
+ * @return array Theme modifications.
  */
 function get_theme_mods() {
 	$theme_slug = get_option( 'stylesheet' );
 	$mods       = get_option( "theme_mods_$theme_slug" );
+
 	if ( false === $mods ) {
 		$theme_name = get_option( 'current_theme' );
 		if ( false === $theme_name ) {
 			$theme_name = wp_get_theme()->get( 'Name' );
 		}
+
 		$mods = get_option( "mods_$theme_name" ); // Deprecated location.
 		if ( is_admin() && false !== $mods ) {
 			update_option( "theme_mods_$theme_slug", $mods );
 			delete_option( "mods_$theme_name" );
 		}
 	}
+
+	if ( ! is_array( $mods ) ) {
+		$mods = array();
+	}
+
 	return $mods;
 }
 
diff --git a/tests/phpunit/tests/option/themeMods.php b/tests/phpunit/tests/option/themeMods.php
index 53683f72a0..2ed14f194f 100644
--- a/tests/phpunit/tests/option/themeMods.php
+++ b/tests/phpunit/tests/option/themeMods.php
@@ -19,6 +19,15 @@ class Tests_Option_Theme_Mods extends WP_UnitTestCase {
 		$this->assertSame( $expected, get_theme_mod( 'test_name' ) );
 	}
 
+	/**
+	 * @ticket 51423
+	 */
+	function test_theme_mod_set_with_invalid_theme_mods_option() {
+		$theme_slug = get_option( 'stylesheet' );
+		update_option( 'theme_mods_' . $theme_slug, '' );
+		self::test_theme_mod_set();
+	}
+
 	function test_theme_mod_update() {
 		set_theme_mod( 'test_update', 'first_value' );
 		$expected = 'updated_value';