diff --git a/jest.config.cjs b/jest.config.cjs
deleted file mode 100644
index 6f3d23e0d..000000000
--- a/jest.config.cjs
+++ /dev/null
@@ -1,31 +0,0 @@
-/** @type {import('jest').Config} */
-const config = {
-  verbose: true,
-  transform: {
-    '^.+\\.m?[tj]sx?$': ['ts-jest'],
-  },
-  testMatch: [
-    '<rootDir>/src/**/__tests__/**/*.ts?(x)',
-    '<rootDir>/(test|src)/**/*(*.)@(spec|test).ts?(x)',
-    '<rootDir>/test/**/*.(test|spec).(js|jsx|ts|tsx)',
-  ],
-  clearMocks: true,
-  collectCoverage: true,
-  coverageReporters: ['json', 'lcov', 'clover', 'cobertura', 'text'],
-  coverageDirectory: 'coverage',
-  coveragePathIgnorePatterns: ['/node_modules/'],
-  testPathIgnorePatterns: ['/node_modules/'],
-  watchPathIgnorePatterns: ['/node_modules/'],
-  reporters: [
-    'default',
-    [
-      'jest-junit',
-      {
-        outputDirectory: 'test-reports',
-      },
-    ],
-  ],
-  preset: 'ts-jest/presets/default-legacy',
-};
-
-module.exports = config;
diff --git a/src/CredentialsClient.ts b/src/CredentialsClient.ts
index 1b79bb16d..f05b2b593 100644
--- a/src/CredentialsClient.ts
+++ b/src/CredentialsClient.ts
@@ -20,14 +20,19 @@ export class CredentialsClient {
   private readonly requestHandler?: NodeHttpHandler;
 
   constructor(props: CredentialsClientProps) {
-    this.region = props.region;
+    if (props.region !== undefined) {
+      this.region = props.region;
+    }
     if (props.proxyServer) {
       info('Configuring proxy handler for STS client');
-      const getProxyForUrl = new ProxyResolver({
+      const proxyOptions: { httpProxy: string; httpsProxy: string; noProxy?: string } = {
         httpProxy: props.proxyServer,
         httpsProxy: props.proxyServer,
-        noProxy: props.noProxy,
-      }).getProxyForUrl;
+      };
+      if (props.noProxy !== undefined) {
+        proxyOptions.noProxy = props.noProxy;
+      }
+      const getProxyForUrl = new ProxyResolver(proxyOptions).getProxyForUrl;
       const handler = new ProxyAgent({ getProxyForUrl });
       this.requestHandler = new NodeHttpHandler({
         httpsAgent: handler,
@@ -38,11 +43,14 @@ export class CredentialsClient {
 
   public get stsClient(): STSClient {
     if (!this._stsClient) {
-      this._stsClient = new STSClient({
-        region: this.region,
-        customUserAgent: USER_AGENT,
-        requestHandler: this.requestHandler ? this.requestHandler : undefined,
-      });
+      const config = { customUserAgent: USER_AGENT } as {
+        customUserAgent: string;
+        region?: string;
+        requestHandler?: NodeHttpHandler;
+      };
+      if (this.region !== undefined) config.region = this.region;
+      if (this.requestHandler !== undefined) config.requestHandler = this.requestHandler;
+      this._stsClient = new STSClient(config);
     }
     return this._stsClient;
   }
@@ -88,9 +96,9 @@ export class CredentialsClient {
   }
 
   private async loadCredentials() {
-    const client = new STSClient({
-      requestHandler: this.requestHandler ? this.requestHandler : undefined,
-    });
+    const config = {} as { requestHandler?: NodeHttpHandler };
+    if (this.requestHandler !== undefined) config.requestHandler = this.requestHandler;
+    const client = new STSClient(config);
     return client.config.credentials();
   }
 }
diff --git a/src/ProxyResolver.ts b/src/ProxyResolver.ts
index d2bc052a2..50291be54 100644
--- a/src/ProxyResolver.ts
+++ b/src/ProxyResolver.ts
@@ -18,9 +18,10 @@ export class ProxyResolver {
     this.options = options;
   }
 
-  getProxyForUrl(url: string, _req: http.ClientRequest): string {
+  // This method matches the interface expected by 'proxy-agent'. It is an arrow function to bind 'this'.
+  public readonly getProxyForUrl = (url: string, _req: http.ClientRequest): string => {
     return this.getProxyForUrlOptions(url, this.options);
-  }
+  };
 
   private getProxyForUrlOptions(url: string | URL, options?: ProxyOptions): string {
     let parsedUrl: URL;
diff --git a/src/helpers.ts b/src/helpers.ts
index 92f29d0db..1babe5861 100644
--- a/src/helpers.ts
+++ b/src/helpers.ts
@@ -115,7 +115,14 @@ export async function getCallerIdentity(client: STSClient): Promise<{ Account: s
   if (!identity.Account || !identity.Arn) {
     throw new Error('Could not get Account ID or ARN from STS. Did you set credentials?');
   }
-  return { Account: identity.Account, Arn: identity.Arn, UserId: identity.UserId };
+  const result: { Account: string; Arn: string; UserId?: string } = {
+    Account: identity.Account,
+    Arn: identity.Arn,
+  };
+  if (identity.UserId !== undefined) {
+    result.UserId = identity.UserId;
+  }
+  return result;
 }
 
 // Obtains account ID from STS Client and sets it as output
diff --git a/src/index.ts b/src/index.ts
index a9f15c45f..20d511e1d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -110,7 +110,10 @@ export async function run() {
     exportRegion(region, outputEnvCredentials);
 
     // Instantiate credentials client
-    const credentialsClient = new CredentialsClient({ region, proxyServer, noProxy });
+    const clientProps: { region: string; proxyServer?: string; noProxy?: string } = { region };
+    if (proxyServer) clientProps.proxyServer = proxyServer;
+    if (noProxy) clientProps.noProxy = noProxy;
+    const credentialsClient = new CredentialsClient(clientProps);
     let sourceAccountId: string;
     let webIdentityToken: string;
 
diff --git a/tsconfig.json b/tsconfig.json
index 227fd8171..9294dbf31 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -3,7 +3,7 @@
     "allowUnreachableCode": false,
     "allowUnusedLabels": false,
     "strict": true,
-    "exactOptionalPropertyTypes": false,
+    "exactOptionalPropertyTypes": true,
     "noFallthroughCasesInSwitch": true,
     "noImplicitOverride": true,
     "noImplicitReturns": true,