diff --git a/src/Plugin/facets/processor/GranularItemProcessor.php b/src/Plugin/facets/processor/GranularItemProcessor.php
index 05c590f..dd74f25 100644
--- a/src/Plugin/facets/processor/GranularItemProcessor.php
+++ b/src/Plugin/facets/processor/GranularItemProcessor.php
@@ -45,6 +45,7 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
 
     $build['granularity'] = [
       '#type' => 'number',
+      '#attributes' => ['min' => 1, 'step' => 1],
       '#title' => $this->t('Granularity'),
       '#default_value' => $configuration['granularity'],
       '#description' => $this->t('The numeric size of the steps to group the result facets in.'),
diff --git a/src/Plugin/facets/query_type/SearchApiGranular.php b/src/Plugin/facets/query_type/SearchApiGranular.php
index 9eed200..34d7bf2 100644
--- a/src/Plugin/facets/query_type/SearchApiGranular.php
+++ b/src/Plugin/facets/query_type/SearchApiGranular.php
@@ -23,7 +23,7 @@ class SearchApiGranular extends QueryTypeRangeBase {
   public function calculateRange($value) {
     return [
       'start' => $value,
-      'stop' => $value + $this->getGranularity(),
+      'stop' => (int) $value + $this->getGranularity(),
     ];
   }
 
@@ -31,6 +31,7 @@ public function calculateRange($value) {
    * {@inheritdoc}
    */
   public function calculateResultFilter($value) {
+    assert($this->getGranularity() > 0);
     return [
       'display' => $value - ($value % $this->getGranularity()),
       'raw' => $value - ($value % $this->getGranularity()) ,