diff --git a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
index 5589dad..3564966 100644
--- a/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
@@ -1654,9 +1654,13 @@ protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $st
     // Define the initial values, if any.
     $initial_value = $initial_value_from_field = [];
     $storage_definition_is_new = empty($this->loadFieldSchemaData($storage_definition));
-    if ($storage_definition_is_new && $storage_definition instanceof BaseFieldDefinition && $table_mapping->allowsSharedTableStorage($storage_definition)) {
+    if ($storage_definition_is_new && $table_mapping->allowsSharedTableStorage($storage_definition)) {
       if (($initial_storage_value = $storage_definition->getInitialValue()) && !empty($initial_storage_value)) {
-        $initial_value = $initial_storage_value[0];
+        // We only support initial values for fields that are stored in shared
+        // tables (i.e. single-value fields).
+        // @todo Implement initial value support for multi-value fields in
+        //   https://www.drupal.org/node/2883851.
+        $initial_value = reset($initial_storage_value);
       }
 
       if ($initial_value_field_name = $storage_definition->getInitialValueFromField()) {