diff --git a/static/js/list_render.js b/static/js/list_render.js
index 02a040c139..ef2a73a61a 100644
--- a/static/js/list_render.js
+++ b/static/js/list_render.js
@@ -4,6 +4,22 @@ const DEFAULTS = {
     instances: new Map(),
 };
 
+
+// ----------------------------------------------------
+// This function describes (programatically) how to use
+// the list_render widget.
+// ----------------------------------------------------
+
+exports.validate_opts = (opts) => {
+    if (opts.html_selector && typeof opts.html_selector !== 'function') {
+        // We have an html_selector, but it is not a function.
+        // This is a programming error.
+        blueslip.error('html_selector should be a function.');
+        return false;
+    }
+    return true;
+};
+
 exports.get_filtered_items = (value, list, opts) => {
     /*
         This is used by the main object (see `create`),
@@ -110,6 +126,10 @@ exports.create = function ($container, list, opts) {
         return;
     }
 
+    if (!exports.validate_opts(opts)) {
+        return;
+    }
+
     if (opts.name && DEFAULTS.instances.get(opts.name)) {
         // Clear event handlers for prior widget.
         const old_widget = DEFAULTS.instances.get(opts.name);