diff --git a/core/modules/field_ui/field_ui.admin.inc b/core/modules/field_ui/field_ui.admin.inc
index a0287bb..f52f208 100644
--- a/core/modules/field_ui/field_ui.admin.inc
+++ b/core/modules/field_ui/field_ui.admin.inc
@@ -519,7 +519,8 @@ function field_ui_field_overview_form($form, &$form_state, $entity_type, $bundle
         '#field_suffix' => '</span>&lrm;',
         '#size' => 15,
         '#description' => t('A unique machine-readable name containing letters, numbers, and underscores.'),
-        '#maxlength' => 32 - drupal_strlen('field_'),
+        // 32 characters minus the 'field_' prefix.
+        '#maxlength' => 26,
         '#prefix' => '<div class="add-new-placeholder">&nbsp;</div>',
         '#machine_name' => array(
           'source' => array('fields', $name, 'label'),