diff -u b/page_manager_ui/src/Form/PageParametersForm.php b/page_manager_ui/src/Form/PageParametersForm.php
--- b/page_manager_ui/src/Form/PageParametersForm.php
+++ b/page_manager_ui/src/Form/PageParametersForm.php
@@ -67,6 +67,12 @@
      */
     foreach ($page->getParameterNames() as $parameter_name) {
       $parameter = $page->getParameter($parameter_name);
+
+      // Use parameter defaults for new parameters.
+      if (empty($parameter)) {
+        $parameter = (array) $page->parameterDefaults();
+      }
+
       $parameter += ['optional' => FALSE];
       $row = [];
       $row['machine_name'] = $parameter['machine_name'];
diff -u b/src/Entity/Page.php b/src/Entity/Page.php
--- b/src/Entity/Page.php
+++ b/src/Entity/Page.php
@@ -250,6 +250,21 @@
   }
 
   /**
+   * Defaults for new parameters.
+   *
+   * @return array
+   *   An array of default parameter values.
+   */
+  public function parameterDefaults() {
+    return [
+      'type' => '',
+      'machine_name' => '',
+      'label' => '',
+      'optional' => FALSE,
+    ];
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function getParameter($name) {