diff --git a/lib/utils.js b/lib/utils.js
index f6e8c705..b1bce5f5 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -454,7 +454,7 @@ var merge = function merge(schema, form, ignore, options, readonly) {
 
     var stdForm = getDefaults(schema, ignore, options);
     // simple case, we have a "*", just put the stdForm there
-    var idx = form.indexOf("*");
+    var idx = form.indexOf && form.indexOf("*") || -1;
     if (idx !== -1) {
         form = form.slice(0, idx).concat(stdForm.form).concat(form.slice(idx + 1));
     }
diff --git a/src/utils.js b/src/utils.js
index 08907253..542b96a6 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -457,7 +457,7 @@ const merge = (schema, form, ignore, options, readonly) => {
 
     const stdForm = getDefaults(schema, ignore, options);
     // simple case, we have a "*", just put the stdForm there
-    const idx = form.indexOf("*");
+    const idx = form.indexOf && form.indexOf("*") || -1;
     if (idx !== -1) {
         form = form
             .slice(0, idx)