diff --git a/src/MoneyBag.php b/src/MoneyBag.php
index 7bce90d..01389bf 100644
--- a/src/MoneyBag.php
+++ b/src/MoneyBag.php
@@ -5,6 +5,7 @@
 namespace Brick\Money;
 
 use Brick\Math\BigRational;
+use Brick\Money\Tests\CurrencyTest;
 
 /**
  * Container for monies in different currencies.
@@ -29,12 +30,12 @@ final class MoneyBag implements MoneyContainer
      */
     public function getAmount($currency) : BigRational
     {
-        if (! $currency instanceof Currency) {
-            $currency = Currency::of($currency);
+        if (is_int($currency)) {
+            $currencyCode = (string) Currency::of($currency);
+        } else {
+            $currencyCode = (string) $currency;
         }
 
-        $currencyCode = (string) $currency;
-
         return isset($this->amounts[$currencyCode])
             ? $this->amounts[$currencyCode]
             : BigRational::zero();
diff --git a/tests/MoneyBagTest.php b/tests/MoneyBagTest.php
index 4fdc7e4..6e1c4b2 100644
--- a/tests/MoneyBagTest.php
+++ b/tests/MoneyBagTest.php
@@ -5,6 +5,7 @@
 namespace Brick\Money\Tests;
 
 use Brick\Money\Context\AutoContext;
+use Brick\Money\Currency;
 use Brick\Money\Money;
 use Brick\Money\MoneyBag;
 use Brick\Money\RationalMoney;
@@ -25,7 +26,7 @@ public function testEmptyMoneyBag() : void
         }
     }
 
-    public function testAddSubtractMoney() : void
+    public function testAddSubtractMoney() : MoneyBag
     {
         $moneyBag = new MoneyBag();
 
@@ -46,5 +47,16 @@ public function testAddSubtractMoney() : void
 
         $moneyBag->add(RationalMoney::of('1/3', 'EUR'));
         $this->assertMoneyBagContains(['EUR' => '21284003/60000', 'JPY' => '4.1234'], $moneyBag);
+
+        return $moneyBag;
+    }
+
+    /**
+     * @depends testAddSubtractMoney
+     */
+    public function testAddCustomCurrency(MoneyBag $moneyBag) : void
+    {
+        $moneyBag->add(Money::of('0.1234', new Currency('BTC', 0, 'Bitcoin', 8)));
+        $this->assertMoneyBagContains(['EUR' => '21284003/60000', 'JPY' => '4.1234', 'BTC' => '0.1234'], $moneyBag);
     }
 }