diff --git a/CRM/Tbusainvoicegen/Timebank.php b/CRM/Tbusainvoicegen/Timebank.php
index c050576..e6fc716 100644
--- a/CRM/Tbusainvoicegen/Timebank.php
+++ b/CRM/Tbusainvoicegen/Timebank.php
@@ -52,11 +52,17 @@ class CRM_Tbusainvoicegen_Timebank {
   private $contactId;
 
   /**
-   * The number of billable members.
+   * The number of members.
    * @var int
    */
   private $memberCount;
 
+  /**
+   * The number of billable members. It's $memberCount -2 with a minimum of zero.
+   * @var int
+   */
+  private $memberCountiBillable;
+
   /**
    * The price for one period of this timebank.
    * @var float
@@ -112,12 +118,15 @@ class CRM_Tbusainvoicegen_Timebank {
   public function __construct($cid, $memberCount) {
     $this->cid = $cid;
     $this->memberCount = $memberCount;
+    $this->memberCountBillable = $memberCount - 2;
+    if ($this->memberCountBillable < 0) {
+      $this->memberCountBillable = 0;
+    }
   }
 
   /**
    * Generate invoices (called from API).
-   * @param int $cid
-   * @param str $billingPeriod The billing period (e.g "2019-2" for the second 2019 payment)
+   * @param array $params An array containing all the values passed into the Invoicegen.generate API.
    * @return array APIv3 standard response.
    */
   public static function generate($params) {
@@ -184,7 +193,7 @@ class CRM_Tbusainvoicegen_Timebank {
     if ($this->price) {
       return $this->price;
     }
-    $adjustedMemberCount = $this->memberCount - 2;
+    $adjustedMemberCount = $this->memberCountBillable;
     foreach ($this->priceArray as $members => $cost) {
       if ($adjustedMemberCount <= $members) {
         break;
@@ -244,7 +253,7 @@ class CRM_Tbusainvoicegen_Timebank {
       self::INVOICEDATE => $this->invoiceDate->format('Y-m-d'),
       self::PERIODBEGIN => $this->periodBegin->format('Y-m-d'),
       self::PERIODEND => $this->periodEnd->format('Y-m-d'),
-      self::BILLABLEMEMBERSCONTRIB => $this->memberCount,
+      self::BILLABLEMEMBERSCONTRIB => $this->memberCountBillable,
       'total_amount' => $this->price,
       'contact_id' => $this->cid,
       'contribution_status_id' => 'Pending',
@@ -347,4 +356,3 @@ class CRM_Tbusainvoicegen_Timebank {
   }
 
 }
-