diff --git a/flow-server/src/main/resources/plugins/application-theme-plugin/theme-generator.js b/flow-server/src/main/resources/plugins/application-theme-plugin/theme-generator.js
index 8f3d3a9e56d..346a7907901 100644
--- a/flow-server/src/main/resources/plugins/application-theme-plugin/theme-generator.js
+++ b/flow-server/src/main/resources/plugins/application-theme-plugin/theme-generator.js
@@ -260,6 +260,7 @@ function generateThemeFile(themeFolder, themeName, themeProperties, productionMo
   themeFile += imports.join('');
   themeFile += `
 window.Vaadin = window.Vaadin || {};
+window.Vaadin.Flow = window.Vaadin.Flow || {};
 window.Vaadin.Flow.injectedGlobalCss = [];
 
 /**