diff --git a/src/Form/RevisionOverviewForm.php b/src/Form/RevisionOverviewForm.php
index 45c2c66..28cdfb0 100755
--- a/src/Form/RevisionOverviewForm.php
+++ b/src/Form/RevisionOverviewForm.php
@@ -369,7 +369,8 @@ class RevisionOverviewForm extends FormBase {
   public function validateForm(array &$form, FormStateInterface $form_state) {
     $input = $form_state->getUserInput();
 
-    if (count($form_state->getValue('node_revisions_table')) <= 1) {
+    $node_revisions = $form_state->getValue('node_revisions_table');
+    if (is_countable($node_revisions) && count($node_revisions) <= 1) {
       $form_state->setErrorByName('node_revisions_table', $this->t('Multiple revisions are needed for comparison.'));
     }
     elseif (!isset($input['radios_left']) || !isset($input['radios_right'])) {