diff --git a/lib/formatters.ts b/lib/formatters.ts
index f88d9df..c14aa52 100644
--- a/lib/formatters.ts
+++ b/lib/formatters.ts
@@ -1,9 +1,9 @@
 export const formatCurrency = (
   number: number,
   currency: string,
-  locale: string | undefined = undefined
+  locale: string | null
 ): string => {
-  const CURRENCY_FORMATTER = new Intl.NumberFormat(locale, {
+  const CURRENCY_FORMATTER = new Intl.NumberFormat(locale || '', {
     currency: currency,
     style: 'currency'
   })