diff --git a/src/mapml-viewer.js b/src/mapml-viewer.js
index c00fe4334..ae2e0609d 100644
--- a/src/mapml-viewer.js
+++ b/src/mapml-viewer.js
@@ -1035,7 +1035,7 @@ export class HTMLMapmlViewerElement extends HTMLElement {
       let queryValue = featureNode.value.value;
 
       if (feature === 'prefers-lang') {
-        return features['prefers-lang'].values.includes(queryValue).toString;
+        return features['prefers-lang'].values.includes(queryValue).toString();
       } else if (feature === 'map-zoom') {
         return evaluateMapZoomFeature(featureNode, this.zoom);
       } else if (feature === 'map-projection') {