diff --git a/CRM/Custom/Form/CustomData.php b/CRM/Custom/Form/CustomData.php
index 91cd7409318b..09cfeec4807f 100644
--- a/CRM/Custom/Form/CustomData.php
+++ b/CRM/Custom/Form/CustomData.php
@@ -38,6 +38,35 @@
  */
 class CRM_Custom_Form_CustomData {
 
+  /**
+   * Generic wrapper to add custom data to a form via a single line in preProcess.
+   * $this->getDefaultEntity() must be defined for the form class for this to work.
+   *
+   * You also have to add something like this to the postProcess:
+   *   $params['custom'] = CRM_Core_BAO_CustomField::postProcess($submitted, $this->_id, $this->getDefaultEntity());
+   *
+   * @param CRM_Core_Form $form
+   * @param null|string $subName
+   * @param null|string $subType
+   * @param null|int $groupCount
+   *
+   * @throws \CRM_Core_Exception
+   */
+  public static function addToForm(&$form, $subName = NULL, $subType = NULL, $groupCount = 1) {
+    $entityName = $form->getDefaultEntity();
+
+    // when custom data is included in this page
+    if (!empty($_POST['hidden_custom'])) {
+      self::preProcess($form, $subName, $subType, $groupCount, $entityName, $form->_id);
+      self::buildQuickForm($form);
+      self::setDefaultValues($form);
+    }
+    // need to assign custom data type and subtype to the template
+    $form->assign('customDataType', $entityName);
+    $form->assign('customDataSubType', $subType);
+    $form->assign('entityID', $form->_id);
+  }
+
   /**
    * @param CRM_Core_Form $form
    * @param null|string $subName
@@ -46,6 +75,8 @@ class CRM_Custom_Form_CustomData {
    * @param string $type
    * @param null|int $entityID
    * @param null $onlySubType
+   *
+   * @throws \CRM_Core_Exception
    */
   public static function preProcess(
     &$form, $subName = NULL, $subType = NULL,
diff --git a/CRM/Member/Form.php b/CRM/Member/Form.php
index 3fad351fffce..39997389e419 100644
--- a/CRM/Member/Form.php
+++ b/CRM/Member/Form.php
@@ -84,6 +84,13 @@ class CRM_Member_Form extends CRM_Contribute_Form_AbstractEditPayment {
    */
   public $_priceSet;
 
+  /**
+   * Explicitly declare the entity api name.
+   */
+  public function getDefaultEntity() {
+    return 'Membership';
+  }
+
   /**
    * Values submitted to the form, processed along the way.
    *
diff --git a/CRM/Member/Form/Membership.php b/CRM/Member/Form/Membership.php
index 6c98758924bb..37fdd965bd82 100644
--- a/CRM/Member/Form/Membership.php
+++ b/CRM/Member/Form/Membership.php
@@ -243,12 +243,9 @@ public function preProcess() {
         $this->_memType = $params['membership_type_id'][1];
       }
     }
-    // when custom data is included in this page
-    if (!empty($_POST['hidden_custom'])) {
-      CRM_Custom_Form_CustomData::preProcess($this, NULL, $this->_memType, 1, 'Membership', $this->_id);
-      CRM_Custom_Form_CustomData::buildQuickForm($this);
-      CRM_Custom_Form_CustomData::setDefaultValues($this);
-    }
+
+    // Add custom data to form
+    CRM_Custom_Form_CustomData::addToForm($this, $this->_memType, 1);
 
     // CRM-4395, get the online pending contribution id.
     $this->_onlinePendingContributionId = NULL;
@@ -455,11 +452,6 @@ public function buildQuickForm() {
       $this->assign('hasPriceSets', $buildPriceSet);
     }
 
-    //need to assign custom data type and subtype to the template
-    $this->assign('customDataType', 'Membership');
-    $this->assign('customDataSubType', $this->_memType);
-    $this->assign('entityID', $this->_id);
-
     if ($this->_action & CRM_Core_Action::DELETE) {
       $this->addButtons(array(
         array(
diff --git a/api/v3/utils.php b/api/v3/utils.php
index 9948ae9e6bdd..a0fb1c67ff69 100644
--- a/api/v3/utils.php
+++ b/api/v3/utils.php
@@ -1117,7 +1117,7 @@ function _civicrm_api3_format_params_for_create(&$params, $entity) {
   }
   $values = array();
   _civicrm_api3_custom_format_params($params, $values, $entity);
-  $params = array_merge($params, $values);
+  $params = CRM_Utils_Array::crmArrayMerge($params, $values);
 }
 
 /**