diff --git a/CRM/Twingle/Profile.php b/CRM/Twingle/Profile.php
index 1a150af..79c11ca 100644
--- a/CRM/Twingle/Profile.php
+++ b/CRM/Twingle/Profile.php
@@ -111,7 +111,8 @@ public function matches($project_id) {
    */
   public function getCustomFieldMapping() {
     $custom_field_mapping = [];
-    if (is_string($custom_field_definition = $this->getAttribute('custom_field_mapping'))) {
+    if ('' !== ($custom_field_definition = $this->getAttribute('custom_field_mapping', ''))) {
+      /** @var string $custom_field_definition */
       $custom_field_maps = preg_split(
         '/\r\n|\r|\n/',
         $custom_field_definition,
@@ -205,7 +206,9 @@ function($project_id) {
    * @return mixed | NULL
    */
   public function getAttribute($attribute_name, $default = NULL) {
-    return $this->data[$attribute_name] ?? $default;
+    return (isset($this->data[$attribute_name]) && $this->data[$attribute_name] !== '')
+      ? $this->data[$attribute_name]
+      : $default;
   }
 
   /**
diff --git a/CRM/Twingle/Submission.php b/CRM/Twingle/Submission.php
index a6ce330..70540ca 100644
--- a/CRM/Twingle/Submission.php
+++ b/CRM/Twingle/Submission.php
@@ -72,8 +72,8 @@ public static function validateSubmission(&$params, $profile = NULL): void {
 
     // Get the payment instrument defined within the profile, or return an error
     // if none matches (i.e. an unknown payment method was submitted).
-    $payment_instrument_id = $profile->getAttribute('pi_' . $params['payment_method']);
-    if (!isset($payment_instrument_id)) {
+    $payment_instrument_id = $profile->getAttribute('pi_' . $params['payment_method'], '');
+    if ('' !== $payment_instrument_id) {
       throw new CRM_Core_Exception(
         E::ts('Payment method could not be matched to existing payment instrument.'),
         'invalid_format'
@@ -101,7 +101,7 @@ public static function validateSubmission(&$params, $profile = NULL): void {
     // matches (i.e. an unknown gender was submitted).
     if (is_string($params['user_gender'])) {
       $gender_id = $profile->getAttribute('gender_' . $params['user_gender']);
-      if (!isset($gender_id)) {
+      if (!is_numeric($gender_id)) {
         throw new CRM_Core_Exception(
           E::ts('Gender could not be matched to existing gender.'),
           'invalid_format'
diff --git a/api/v3/TwingleDonation/Submit.php b/api/v3/TwingleDonation/Submit.php
index ecaeac8..e24cb2f 100644
--- a/api/v3/TwingleDonation/Submit.php
+++ b/api/v3/TwingleDonation/Submit.php
@@ -420,7 +420,7 @@ function civicrm_api3_twingle_donation_Submit($params) {
       // Get the prefix ID defined within the profile
       if (
         isset($params['user_gender'])
-        && NULL !== ($prefix_id = $profile->getAttribute('prefix_' . $params['user_gender']))
+        && is_numeric($prefix_id = $profile->getAttribute('prefix_' . $params['user_gender']))
       ) {
         $contact_data['prefix_id'] = $prefix_id;
       }
@@ -830,8 +830,8 @@ function civicrm_api3_twingle_donation_Submit($params) {
       $result_values['membership'] = $membership;
 
       // call the postprocess API
-      $postprocess_call = $profile->getAttribute('membership_postprocess_call');
-      if (is_string($postprocess_call)) {
+      if ('' !== ($postprocess_call = $profile->getAttribute('membership_postprocess_call', ''))) {
+        /** @var string $postprocess_call */
         [$pp_entity, $pp_action] = explode('.', $postprocess_call, 2);
         try {
           // gather the contribution IDs