diff --git a/src/interfaces/parser.ts b/src/interfaces/parser.ts
index 7d663573..ea111027 100644
--- a/src/interfaces/parser.ts
+++ b/src/interfaces/parser.ts
@@ -118,7 +118,7 @@ export type FlagProps = {
   /**
    * Accept an environment variable as input
    */
-   env?: string;
+  env?: string;
   /**
    * If true, the flag will not be shown in the help.
    */
@@ -142,7 +142,7 @@ export type FlagProps = {
   /**
    * Define complex relationships between flags.
    */
-   relationships?: Relationship[];
+  relationships?: Relationship[];
 }
 
 export type BooleanFlagProps = FlagProps & {
diff --git a/src/parser/parse.ts b/src/parser/parse.ts
index a7d965e2..bee68e18 100644
--- a/src/parser/parse.ts
+++ b/src/parser/parse.ts
@@ -258,7 +258,7 @@ export class Parser<T extends ParserInput, TFlags extends OutputFlags<T['flags']
         stdinRead = true
       }
 
-      if (!args[i] && 'default' in arg) {
+      if (!args[i] && arg?.default !== undefined) {
         if (typeof arg.default === 'function') {
           // eslint-disable-next-line no-await-in-loop
           const f = await arg.default()
diff --git a/test/parser/parse.test.ts b/test/parser/parse.test.ts
index 45d14031..48a27b5d 100644
--- a/test/parser/parse.test.ts
+++ b/test/parser/parse.test.ts
@@ -7,6 +7,7 @@ import {Interfaces} from '../../src'
 import {URL} from 'url'
 import {directory, file} from '../../src/parser/flags'
 import * as sinon from 'sinon'
+import {CLIError} from '../../src/errors'
 
 const stripAnsi = require('strip-ansi')
 
@@ -36,6 +37,16 @@ describe('parse', () => {
     expect(out.args).to.deep.equal({foo: 'arg1', bar: 'arg2'})
   })
 
+  it('should throw if unexpected argument is provided', async () => {
+    try {
+      await parse(['arg1'], {})
+      expect.fail('should have thrown')
+    } catch (error) {
+      const err = error as CLIError
+      expect(err.message).to.include('Unexpected argument: arg1')
+    }
+  })
+
   describe('output: array', () => {
     it('--bool', async () => {
       const out = await parse(['--bool'], {