diff --git a/src/lib/index.ts b/src/lib/index.ts
index 515d26f..67434db 100644
--- a/src/lib/index.ts
+++ b/src/lib/index.ts
@@ -417,8 +417,11 @@ export function format(
 	css: string,
 	{ minify = false, tab_size = undefined }: FormatOptions = Object.create(null),
 ): string {
-	if (tab_size !== undefined && Number(tab_size) < 1) {
-		throw new TypeError('tab_size must be a number greater than 0')
+	if (tab_size !== undefined) {
+		let normalized = Number(tab_size)
+		// An invalid tab_size (non-numeric, fractional, NaN, Infinity, < 1) falls
+		// back to the default tab indentation instead of throwing.
+		tab_size = Number.isInteger(normalized) && normalized >= 1 ? normalized : undefined
 	}
 
 	const NEWLINE = minify ? EMPTY_STRING : '\n'
diff --git a/test/tab-size.test.ts b/test/tab-size.test.ts
index 6332db4..1a2f17a 100644
--- a/test/tab-size.test.ts
+++ b/test/tab-size.test.ts
@@ -34,14 +34,35 @@ test('tab_size: 2', () => {
 	expect(actual).toEqual(expected)
 })
 
-test('invalid tab_size: 0', () => {
-	expect(() => format(fixture, { tab_size: 0 })).toThrow('tab_size must be a number greater than 0')
+let default_indentation = format(fixture)
+
+test('invalid tab_size: 0 falls back to default tab indentation', () => {
+	expect(format(fixture, { tab_size: 0 })).toEqual(default_indentation)
 })
 
-test('invalid tab_size: negative', () => {
-	expect(() => format(fixture, { tab_size: -1 })).toThrow(
-		'tab_size must be a number greater than 0',
-	)
+test('invalid tab_size: negative falls back to default tab indentation', () => {
+	expect(format(fixture, { tab_size: -1 })).toEqual(default_indentation)
+})
+
+test('invalid tab_size: fractional falls back to default tab indentation', () => {
+	expect(format(fixture, { tab_size: 2.5 })).toEqual(default_indentation)
+})
+
+test('invalid tab_size: NaN falls back to default tab indentation', () => {
+	expect(format(fixture, { tab_size: NaN })).toEqual(default_indentation)
+})
+
+test('invalid tab_size: Infinity falls back to default tab indentation', () => {
+	expect(format(fixture, { tab_size: Infinity })).toEqual(default_indentation)
+})
+
+test('invalid tab_size: non-numeric string falls back to default tab indentation (bypassing TypeScript types)', () => {
+	// @ts-expect-error tab_size is typed as number, but JS callers can pass anything at runtime
+	expect(format(fixture, { tab_size: 'abc' })).toEqual(default_indentation)
+})
+
+test('invalid tab_size does not throw', () => {
+	expect(() => format(fixture, { tab_size: 0 }), 'invalid tab_size should not throw').not.toThrow()
 })
 
 test('combine tab_size and minify', () => {