diff --git a/main.js b/main.js
index e1ce3a6..c535e2a 100644
--- a/main.js
+++ b/main.js
@@ -164,6 +164,11 @@ const sourceInput = customLayerDiv.querySelector('input[name=source]');
 const colorInput = customLayerDiv.querySelector('input[name=color]');
 
 customLayerDiv.querySelector("button").addEventListener("click", function(){
+  if (!sourceInput.value.toLowerCase().endsWith(".geojson")) {
+    if (!confirm("Input URL doesn't end in .geojson, so is probably not a valid GeoJSON file. Do you want to continue anyway?")) {
+      return;
+    }
+  }
   newCustomLayer(labelInput.value, sourceInput.value, colorInput.value);
 });
 document.querySelector("aside").appendChild(customLayerDiv);