diff --git a/tests/phpunit/tests/ajax/wpAjaxUpdateTheme.php b/tests/phpunit/tests/ajax/wpAjaxUpdateTheme.php
index c424d7f582..610cafb169 100644
--- a/tests/phpunit/tests/ajax/wpAjaxUpdateTheme.php
+++ b/tests/phpunit/tests/ajax/wpAjaxUpdateTheme.php
@@ -152,6 +152,9 @@ class Tests_Ajax_wpAjaxUpdateTheme extends WP_Ajax_UnitTestCase {
 		$_POST['_ajax_nonce'] = wp_create_nonce( 'updates' );
 		$_POST['slug']        = 'camelCase';
 
+		// Prevent wp_update_themes() from running.
+		wp_installing( true );
+
 		// Make the request.
 		try {
 			$this->_handleAjax( 'update-theme' );
@@ -159,6 +162,8 @@ class Tests_Ajax_wpAjaxUpdateTheme extends WP_Ajax_UnitTestCase {
 			unset( $e );
 		}
 
+		wp_installing( false );
+
 		// Get the response.
 		$response = json_decode( $this->_last_response, true );