diff --git a/index.d.ts b/index.d.ts
index e92d14a..75ceae5 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -25,7 +25,8 @@ declare namespace meow {
 	type BooleanFlag = Flag<'boolean', boolean>;
 	type NumberFlag = Flag<'number', number>;
 
-	type AnyFlags = {[key: string]: StringFlag | BooleanFlag | NumberFlag};
+	type AnyFlag = StringFlag | BooleanFlag | NumberFlag;
+	type AnyFlags = {[key: string]: AnyFlag};
 
 	interface Options<Flags extends AnyFlags> {
 		/**
@@ -195,14 +196,26 @@ declare namespace meow {
 		readonly hardRejection?: boolean;
 	}
 
-	type TypedFlags<Flags extends AnyFlags> = {
-		[F in keyof Flags]: Flags[F] extends {type: 'number'}
+	type TypedFlag<Flag extends AnyFlag> =
+		Flag extends {type: 'number'}
 			? number
-			: Flags[F] extends {type: 'string'}
+			: Flag extends {type: 'string'}
 				? string
-				: Flags[F] extends {type: 'boolean'}
+				: Flag extends {type: 'boolean'}
 					? boolean
 					: unknown;
+
+	type PossiblyOptionalFlag<Flag extends AnyFlag, FlagType> =
+		Flag extends {isRequired: true}
+			? FlagType
+			: Flag extends {default: any}
+				? FlagType
+				: FlagType | undefined;
+
+	type TypedFlags<Flags extends AnyFlags> = {
+		[F in keyof Flags]: Flags[F] extends {isMultiple: true}
+			? PossiblyOptionalFlag<Flags[F], Array<TypedFlag<Flags[F]>>>
+			: PossiblyOptionalFlag<Flags[F], TypedFlag<Flags[F]>>
 	};
 
 	interface Result<Flags extends AnyFlags> {
diff --git a/index.test-d.ts b/index.test-d.ts
index 640b8ca..c3003e8 100644
--- a/index.test-d.ts
+++ b/index.test-d.ts
@@ -6,12 +6,21 @@ import {Result} from '.';
 expectType<Result<never>>(meow('Help text'));
 expectType<Result<never>>(meow('Help text', {hardRejection: false}));
 expectAssignable<{flags: {foo: number}}>(
-	meow({flags: {foo: {type: 'number'}}})
+	meow({flags: {foo: {type: 'number', isRequired: true}}})
 );
 expectAssignable<{flags: {foo: string}}>(
-	meow({flags: {foo: {type: 'string'}}})
+	meow({flags: {foo: {type: 'string', isRequired: true}}})
 );
 expectAssignable<{flags: {foo: boolean}}>(
+	meow({flags: {foo: {type: 'boolean', isRequired: true}}})
+);
+expectAssignable<{flags: {foo: number | undefined}}>(
+	meow({flags: {foo: {type: 'number'}}})
+);
+expectAssignable<{flags: {foo: string | undefined}}>(
+	meow({flags: {foo: {type: 'string'}}})
+);
+expectAssignable<{flags: {foo: boolean | undefined}}>(
 	meow({flags: {foo: {type: 'boolean'}}})
 );
 expectType<Result<never>>(meow({description: 'foo'}));
@@ -34,7 +43,8 @@ const result = meow('Help text', {
 	flags: {
 		foo: {type: 'boolean', alias: 'f'},
 		'foo-bar': {type: 'number'},
-		bar: {type: 'string', default: ''}
+		bar: {type: 'string', default: ''},
+		abc: {type: 'string', isMultiple: true}
 	}
 });
 
@@ -42,13 +52,15 @@ expectType<string[]>(result.input);
 expectType<PackageJson>(result.pkg);
 expectType<string>(result.help);
 
-expectType<boolean>(result.flags.foo);
+expectType<boolean | undefined>(result.flags.foo);
 expectType<unknown>(result.flags.fooBar);
 expectType<string>(result.flags.bar);
-expectType<boolean>(result.unnormalizedFlags.foo);
+expectType<string[] | undefined>(result.flags.abc);
+expectType<boolean | undefined>(result.unnormalizedFlags.foo);
 expectType<unknown>(result.unnormalizedFlags.f);
-expectType<number>(result.unnormalizedFlags['foo-bar']);
+expectType<number | undefined>(result.unnormalizedFlags['foo-bar']);
 expectType<string>(result.unnormalizedFlags.bar);
+expectType<string[] | undefined>(result.unnormalizedFlags.abc);
 
 result.showHelp();
 result.showHelp(1);
diff --git a/test/test.js b/test/test.js
index 44e34a0..e1f329a 100644
--- a/test/test.js
+++ b/test/test.js
@@ -510,6 +510,17 @@ test('isMultiple - handles multi-word flag name', t => {
 	});
 });
 
+test('isMultiple - handles non-set flags correctly', t => {
+	t.deepEqual(meow({
+		argv: [],
+		flags: {
+			foo: {
+				isMultiple: true
+			}
+		}
+	}).flags, {});
+});
+
 if (NODE_MAJOR_VERSION >= 14) {
 	test('supports es modules', async t => {
 		try {
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..720afed
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,19 @@
+{
+	"files": [
+		"index.d.ts",
+		"index.test-d.ts",
+	],
+	"compilerOptions": {
+		"strict": true,
+		"jsx": "react",
+		"target": "es2018",
+		"lib": [
+			"es2018"
+		],
+		"module": "commonjs",
+		"moduleResolution": "node",
+		"noImplicitAny": true,
+		"strictNullChecks": true,
+		"noUnusedLocals": true
+	}
+}