diff --git a/package.json b/package.json
index 129ff65..867a33a 100644
--- a/package.json
+++ b/package.json
@@ -40,8 +40,11 @@
     }
   },
   "scripts": {
-    "build": "rm -rf dist && tsc -p tsconfig.json && tsc -p tsconfig-cjs.json && ./fixup.sh",
+    "build": "rm -rf dist && tsc -p tsconfig.json && tsc -p tsconfig.cjs.json && ./fixup.sh",
     "dev": "tsc --watch",
     "prepare": "npm run build"
+  },
+  "directories": {
+    "test": "test"
   }
 }
diff --git a/src/index.ts b/src/index.ts
index d8a0139..d05f8af 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -11,7 +11,7 @@ interface YFResponse {
   chart: { result: { meta: { regularMarketPrice: number } }[] }
 }
 
-export async function getExchangeRate (from: CurrencyCode, to: CurrencyCode): Promise<number | undefined> {
+export async function getExchangeRate (from: CurrencyCode, to: CurrencyCode): Promise<number> {
   let response
   try {
     const rateUrl = YF_BASE + from.toUpperCase() + to.toUpperCase() + YF_PARAMS
@@ -26,8 +26,10 @@ export async function getExchangeRate (from: CurrencyCode, to: CurrencyCode): Pr
 
   const result = await response.json() as YFResponse
   const rate = result.chart?.result[0]?.meta?.regularMarketPrice
+
   if (!rate) {
     throw new MalformedError('Service did not return correct data structure.')
   }
+
   return rate
 }
diff --git a/tsconfig-base.json b/tsconfig.base.json
similarity index 100%
rename from tsconfig-base.json
rename to tsconfig.base.json
diff --git a/tsconfig-cjs.json b/tsconfig.cjs.json
similarity index 78%
rename from tsconfig-cjs.json
rename to tsconfig.cjs.json
index 4949551..2ed8ed8 100644
--- a/tsconfig-cjs.json
+++ b/tsconfig.cjs.json
@@ -1,5 +1,5 @@
 {
-  "extends": "./tsconfig-base.json",
+  "extends": "./tsconfig.base.json",
   "compilerOptions": {
     "declaration": false,
     "module": "commonjs",
diff --git a/tsconfig.json b/tsconfig.json
index 3228364..df6e956 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,5 +1,5 @@
 {
-  "extends": "./tsconfig-base.json",
+  "extends": "./tsconfig.base.json",
   "compilerOptions": {
     "module": "esnext",
     "outDir": "dist/esm",