diff --git a/includes/class-api.php b/includes/class-api.php
index 86bd3a2f..14377acb 100644
--- a/includes/class-api.php
+++ b/includes/class-api.php
@@ -1036,7 +1036,12 @@ public function set_contact_properties( $contact, $user_data, $form_id, $updated
 					$original_field_data = $this->plugin->get_process_form()->get_original_fields( $form_id );
 					$custom_field_name   = '';
 					$should_include      = apply_filters( 'constant_contact_include_custom_field_label', false, $form_id );
-					$custom_field        = ( $original_field_data[ $original ] );
+					$custom_field        = $original_field_data[ $original ] ?? null;
+
+					if ( ! is_array( $custom_field ) || empty( $custom_field['name'] ) ) {
+						break; // Queued request references a form field that no longer exists on the form.
+					}
+
 					$new_custom_field    = '';
 					$contact['custom_fields'] = [];
 					// @todo Fix me.