diff --git a/index.d.ts b/index.d.ts
index 9602f55..e4e2202 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -125,7 +125,11 @@ declare namespace meow {
 		readonly inferType?: boolean;
 
 		/**
-		Value of `boolean` flags not defined in `argv`. If set to `undefined` the flags not defined in `argv` will be excluded from the result. The `default` value set in `boolean` flags take precedence over `booleanDefault`.
+		Value of `boolean` flags not defined in `argv`.
+
+		If set to `undefined` the flags not defined in `argv` will be excluded from the result. The `default` value set in `boolean` flags take precedence over `booleanDefault`.
+
+		_Note: If used in conjunction with `isMultiple`, the default flag value is set to `[]`._
 
 		__Caution: Explicitly specifying undefined for `booleanDefault` has different meaning from omitting key itself.__
 
@@ -198,24 +202,24 @@ declare namespace meow {
 
 	type TypedFlag<Flag extends AnyFlag> =
 		Flag extends {type: 'number'}
-			? number
-			: Flag extends {type: 'string'}
-				? string
-				: Flag extends {type: 'boolean'}
-					? boolean
-					: unknown;
+		? number
+		: Flag extends {type: 'string'}
+		? string
+		: Flag extends {type: 'boolean'}
+		? boolean
+		: unknown;
 
 	type PossiblyOptionalFlag<Flag extends AnyFlag, FlagType> =
 		Flag extends {isRequired: true}
-			? FlagType
-			: Flag extends {default: any}
-				? FlagType
-				: FlagType | undefined;
+		? 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]>>
+		? PossiblyOptionalFlag<Flags[F], Array<TypedFlag<Flags[F]>>>
+		: PossiblyOptionalFlag<Flags[F], TypedFlag<Flags[F]>>
 	};
 
 	interface Result<Flags extends AnyFlags> {
@@ -269,14 +273,14 @@ import foo = require('.');
 
 const cli = meow(`
 	Usage
-	  $ foo <input>
+		$ foo <input>
 
 	Options
-	  --rainbow, -r  Include a rainbow
+		--rainbow, -r  Include a rainbow
 
 	Examples
-	  $ foo unicorns --rainbow
-	  🌈 unicorns 🌈
+		$ foo unicorns --rainbow
+		🌈 unicorns 🌈
 `, {
 	flags: {
 		rainbow: {
diff --git a/readme.md b/readme.md
index dd88d4c..153993a 100644
--- a/readme.md
+++ b/readme.md
@@ -239,9 +239,12 @@ Type: `boolean | null | undefined`\
 Default: `false`
 
 Value of `boolean` flags not defined in `argv`.
+
 If set to `undefined` the flags not defined in `argv` will be excluded from the result.
 The `default` value set in `boolean` flags take precedence over `booleanDefault`.
 
+_Note: If used in conjunction with `isMultiple`, the default flag value is set to `[]`._
+
 Example:
 
 ```js