diff --git a/index.d.ts b/index.d.ts
index d8da3ce..aed9fdf 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -2,7 +2,7 @@ export interface Options {
 	/**
 	Count [ambiguous width characters](https://www.unicode.org/reports/tr11/#Ambiguous) as having narrow width (count of 1) instead of wide width (count of 2).
 
-	@default false
+	@default true
 	*/
 	readonly ambiguousIsNarrow: boolean;
 }
diff --git a/index.js b/index.js
index 35882b0..23f3f9a 100644
--- a/index.js
+++ b/index.js
@@ -7,6 +7,11 @@ export default function stringWidth(string, options = {}) {
 		return 0;
 	}
 
+	options = {
+		ambiguousIsNarrow: true,
+		...options
+	};
+
 	string = stripAnsi(string);
 
 	if (string.length === 0) {
@@ -15,7 +20,7 @@ export default function stringWidth(string, options = {}) {
 
 	string = string.replace(emojiRegex(), '  ');
 
-	const ambiguousCharWidth = options.ambiguousIsNarrow ? 1 : 2;
+	const ambiguousCharacterWidth = options.ambiguousIsNarrow ? 1 : 2;
 	let width = 0;
 
 	for (let index = 0; index < string.length; index++) {
@@ -38,7 +43,7 @@ export default function stringWidth(string, options = {}) {
 				width += 2;
 				break;
 			case 'A':
-				width += ambiguousCharWidth;
+				width += ambiguousCharacterWidth;
 				break;
 			default:
 				width += 1;
diff --git a/test.js b/test.js
index c12fa8e..41b7181 100644
--- a/test.js
+++ b/test.js
@@ -5,8 +5,9 @@ test('main', t => {
 	t.is(stringWidth('abcde'), 5);
 	t.is(stringWidth('古池や'), 6);
 	t.is(stringWidth('あいうabc'), 9);
-	t.is(stringWidth('あいう★'), 8);
-	t.is(stringWidth('あいう★', {ambiguousIsNarrow: true}), 7);
+	t.is(stringWidth('あいう★'), 7);
+	t.is(stringWidth('あいう★', {ambiguousIsNarrow: false}), 8);
+	t.is(stringWidth('±'), 1);
 	t.is(stringWidth('ノード.js'), 9);
 	t.is(stringWidth('你好'), 4);
 	t.is(stringWidth('안녕하세요'), 10);