diff --git a/core/modules/field_ui/src/Form/FieldStorageAddForm.php b/core/modules/field_ui/src/Form/FieldStorageAddForm.php
index 7c54ca94cc..899eca9ed5 100644
--- a/core/modules/field_ui/src/Form/FieldStorageAddForm.php
+++ b/core/modules/field_ui/src/Form/FieldStorageAddForm.php
@@ -192,7 +192,7 @@ public function buildForm(array $form, FormStateInterface $form_state, $entity_t
       '#description' => $this->t('A unique machine-readable name containing letters, numbers, and underscores.'),
       // Calculate characters depending on the length of the field prefix
       // setting. Maximum length is 32.
-      '#maxlength' => FieldStorageConfig::NAME_MAX_LENGTH - strlen($field_prefix),
+      '#maxlength' => FieldStorageConfig::NAME_MAX_LENGTH - ($field_prefix ? strlen($field_prefix) : 0),
       '#machine_name' => [
         'source' => ['new_storage_wrapper', 'label'],
         'exists' => [$this, 'fieldNameExists'],