diff --git a/src/components/structures/UserMenuButton.tsx b/src/components/structures/UserMenuButton.tsx
index d8f96d4a91..5fbab796a6 100644
--- a/src/components/structures/UserMenuButton.tsx
+++ b/src/components/structures/UserMenuButton.tsx
@@ -80,7 +80,7 @@ export default class UserMenuButton extends React.Component<IProps, IState> {
     private isUserOnDarkTheme(): boolean {
         const theme = SettingsStore.getValue("theme");
         if (theme.startsWith("custom-")) {
-            return getCustomTheme(theme.substring(0, 7)).is_dark;
+            return getCustomTheme(theme.substring("custom-".length)).is_dark;
         }
         return theme === "dark";
     }