diff --git a/src/wp-includes/class-wp-theme.php b/src/wp-includes/class-wp-theme.php
index f9f93994e7..a8975457d2 100644
--- a/src/wp-includes/class-wp-theme.php
+++ b/src/wp-includes/class-wp-theme.php
@@ -242,6 +242,9 @@ final class WP_Theme implements ArrayAccess {
 			}
 		}
 
+		// Handle a numeric theme directory as a string.
+		$theme_dir = (string) $theme_dir;
+
 		$this->theme_root = $theme_root;
 		$this->stylesheet = $theme_dir;
 
diff --git a/tests/phpunit/tests/theme/wpTheme.php b/tests/phpunit/tests/theme/wpTheme.php
index 4f2fec9f2b..bd04148721 100644
--- a/tests/phpunit/tests/theme/wpTheme.php
+++ b/tests/phpunit/tests/theme/wpTheme.php
@@ -112,6 +112,20 @@ class Tests_Theme_wpTheme extends WP_UnitTestCase {
 		$this->assertSame( 'subdir/theme2', $theme->get_template() );
 	}
 
+	/**
+	 * Tests that WP_Theme::__construct() handles a numeric theme directory as a string.
+	 *
+	 * @ticket 54645
+	 *
+	 * @covers WP_Theme::__construct
+	 */
+	public function test_new_WP_Theme_numeric_theme_directory() {
+		$theme = new WP_Theme( 1234, $this->theme_root );
+
+		$this->assertSame( '1234', $theme->get_stylesheet(), 'The stylesheet property should be a string.' );
+		$this->assertSame( '1234', $theme->get_template(), 'The template property should be a string.' );
+	}
+
 	/**
 	 * @ticket 21749
 	 */