diff -u b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
--- b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -189,7 +189,10 @@
         $entity_id = $this->get('entity')->getTargetIdentifier();
         // If the entity has been saved and we're trying to set both the
         // target_id and the entity values with a non-null target ID, then the
-        // value for target_id should match the ID of the entity value.
+        // value for target_id should match the ID of the entity value. The
+        // entity ID as returned by $entity->id() might be a string, but the
+        // provided target_id might be an integer - therefore we have to do a
+        // non-strict comparison.
         if (!$this->entity->isNew() && $values['target_id'] !== NULL && ($entity_id != $values['target_id'])) {
           throw new \InvalidArgumentException('The target id and entity passed to the entity reference item do not match.');
         }