diff --git a/src/core.ts b/src/core.ts
index ab80c666..add3d06d 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -961,14 +961,16 @@ export const ensurePresent = <T>(value: T | null | undefined): T => {
 /**
  * Read an environment variable.
  *
+ * Trims beginning and trailing whitespace.
+ *
  * Will return undefined if the environment variable doesn't exist or cannot be accessed.
  */
 export const readEnv = (env: string): string | undefined => {
   if (typeof process !== 'undefined') {
-    return process.env?.[env] ?? undefined;
+    return process.env?.[env]?.trim() ?? undefined;
   }
   if (typeof Deno !== 'undefined') {
-    return Deno.env?.get?.(env);
+    return Deno.env?.get?.(env)?.trim();
   }
   return undefined;
 };
diff --git a/src/index.ts b/src/index.ts
index 9cc56814..500271e4 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -104,7 +104,7 @@ export class Anthropic extends Core.APIClient {
       apiKey,
       authToken,
       ...opts,
-      baseURL: baseURL ?? `https://api.anthropic.com`,
+      baseURL: baseURL || `https://api.anthropic.com`,
     };
 
     super({
diff --git a/tests/index.test.ts b/tests/index.test.ts
index 7b72baf9..67e94353 100644
--- a/tests/index.test.ts
+++ b/tests/index.test.ts
@@ -140,7 +140,7 @@ describe('instantiate client', () => {
     });
 
     afterEach(() => {
-      process.env['SINK_BASE_URL'] = undefined;
+      process.env['ANTHROPIC_BASE_URL'] = undefined;
     });
 
     test('explicit option', () => {
@@ -153,6 +153,18 @@ describe('instantiate client', () => {
       const client = new Anthropic({ apiKey: 'my-anthropic-api-key' });
       expect(client.baseURL).toEqual('https://example.com/from_env');
     });
+
+    test('empty env variable', () => {
+      process.env['ANTHROPIC_BASE_URL'] = ''; // empty
+      const client = new Anthropic({ apiKey: 'my-anthropic-api-key' });
+      expect(client.baseURL).toEqual('https://api.anthropic.com');
+    });
+
+    test('blank env variable', () => {
+      process.env['ANTHROPIC_BASE_URL'] = '  '; // blank
+      const client = new Anthropic({ apiKey: 'my-anthropic-api-key' });
+      expect(client.baseURL).toEqual('https://api.anthropic.com');
+    });
   });
 
   test('maxRetries option is correctly set', () => {