diff --git a/loko/assets/static/assets/unified_index.js b/loko/assets/static/assets/unified_index.js
index 21cf7b9..9280376 100644
--- a/loko/assets/static/assets/unified_index.js
+++ b/loko/assets/static/assets/unified_index.js
@@ -53,6 +53,7 @@ var assetsLoadModes = {};     // thematic_code → null | 'always' | 'filter' |
 
 // MapLibre sources
 var mergedLocationsGeojson = { type: 'FeatureCollection', features: [] };
+var mergedManagementZonesGeojson = { type: 'FeatureCollection', features: [] };
 var mergedAssetsGeojson = { type: 'FeatureCollection', features: [] };
 var currentFetchAbortController = null;
 var assetClickPopup = null;
@@ -2805,7 +2806,7 @@ function rebuildMapSources() {
     const locSrc = map.getSource('locations');
     const asstSrc = map.getSource('assets');
     const zoneSrc = map.getSource('management_zones');
-    if (!locSrc || !asstSrc) return;   // sources not yet added (before mapLoaded)
+    if (!locSrc || !asstSrc || !zoneSrc) return;   // sources not yet added (before mapLoaded)
 
     mergedLocationsGeojson = {
         type: 'FeatureCollection',