diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef5ba2307fd2..717bce08fede 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## Unreleased
 
 * Exclude global variables from `$capture_state` ([#4463](https://github.com/sveltejs/svelte/issues/4463))
+* Fix bitmask overflow for slots ([#4481](https://github.com/sveltejs/svelte/issues/4481))
 
 ## 3.19.1
 
diff --git a/src/runtime/internal/utils.ts b/src/runtime/internal/utils.ts
index 4ba7e18c153a..acb88c766945 100644
--- a/src/runtime/internal/utils.ts
+++ b/src/runtime/internal/utils.ts
@@ -83,7 +83,11 @@ export function get_slot_changes(definition, $$scope, dirty, fn) {
 	if (definition[2] && fn) {
 		const lets = definition[2](fn(dirty));
 
-		if (typeof $$scope.dirty === 'object') {
+		if ($$scope.dirty === undefined) {
+			return lets;
+		}
+
+		if (typeof lets === 'object') {
 			const merged = [];
 			const len = Math.max($$scope.dirty.length, lets.length);
 			for (let i = 0; i < len; i += 1) {
diff --git a/test/runtime/samples/bitmask-overflow-slot-3/Echo.svelte b/test/runtime/samples/bitmask-overflow-slot-3/Echo.svelte
new file mode 100644
index 000000000000..d3ecf142c924
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-3/Echo.svelte
@@ -0,0 +1,9 @@
+<script>
+	let dummy = 0;
+	function increment () {
+		dummy = 1;
+	}
+</script>
+
+<slot dummy={dummy}></slot>
+<button on:click={increment}></button>
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot-3/_config.js b/test/runtime/samples/bitmask-overflow-slot-3/_config.js
new file mode 100644
index 000000000000..93e548e5f7ef
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-3/_config.js
@@ -0,0 +1,30 @@
+export default {
+
+	html: `
+		<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+		<p>0</p>
+		<button></button>
+	`,
+
+	async test({ assert, component, target, window }) {
+		// change from inside
+		const button = target.querySelector('button');
+		await button.dispatchEvent(new window.MouseEvent('click'));
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+			<p>1</p>
+			<button></button>
+		`);
+
+		// change from outside
+		component._0 = 'a';
+		component._40 = 'b';
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>a_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39b</p>
+			<p>1</p>
+			<button></button>
+		`);	
+	}
+};
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot-3/main.svelte b/test/runtime/samples/bitmask-overflow-slot-3/main.svelte
new file mode 100644
index 000000000000..ae798e4aeeb5
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-3/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	import Echo from './Echo.svelte';
+
+	export let _0 = '_0', _1 = '_1', _2 = '_2', _3 = '_3', _4 = '_4', _5 = '_5', _6 = '_6', _7 = '_7', _8 = '_8', _9 = '_9', _10 = '_10', _11 = '_11', _12 = '_12', _13 = '_13', _14 = '_14', _15 = '_15', _16 = '_16', _17 = '_17', _18 = '_18', _19 = '_19', _20 = '_20', _21 = '_21', _22 = '_22', _23 = '_23', _24 = '_24', _25 = '_25', _26 = '_26', _27 = '_27', _28 = '_28', _29 = '_29', _30 = '_30', _31 = '_31', _32 = '_32', _33 = '_33', _34 = '_34', _35 = '_35', _36 = '_36', _37 = '_37', _38 = '_38', _39 = '_39', _40 = '_40';
+</script>
+
+<Echo let:dummy>
+	<p>{_0}{_1}{_2}{_3}{_4}{_5}{_6}{_7}{_8}{_9}{_10}{_11}{_12}{_13}{_14}{_15}{_16}{_17}{_18}{_19}{_20}{_21}{_22}{_23}{_24}{_25}{_26}{_27}{_28}{_29}{_30}{_31}{_32}{_33}{_34}{_35}{_36}{_37}{_38}{_39}{_40}</p>
+	<p>{dummy}</p>
+</Echo>
+
diff --git a/test/runtime/samples/bitmask-overflow-slot-4/Echo.svelte b/test/runtime/samples/bitmask-overflow-slot-4/Echo.svelte
new file mode 100644
index 000000000000..2e1beda492d9
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-4/Echo.svelte
@@ -0,0 +1,11 @@
+<script>
+	export let _0 = '_0', _1 = '_1', _2 = '_2', _3 = '_3', _4 = '_4', _5 = '_5', _6 = '_6', _7 = '_7', _8 = '_8', _9 = '_9', _10 = '_10', _11 = '_11', _12 = '_12', _13 = '_13', _14 = '_14', _15 = '_15', _16 = '_16', _17 = '_17', _18 = '_18', _19 = '_19', _20 = '_20', _21 = '_21', _22 = '_22', _23 = '_23', _24 = '_24', _25 = '_25', _26 = '_26', _27 = '_27', _28 = '_28', _29 = '_29', _30 = '_30', _31 = '_31', _32 = '_32', _33 = '_33', _34 = '_34', _35 = '_35', _36 = '_36', _37 = '_37', _38 = '_38', _39 = '_39', _40 = '_40';	
+	let dummy = 0;
+	function increment () {
+		dummy = 1;
+	}
+</script>
+
+<p>{_0}{_1}{_2}{_3}{_4}{_5}{_6}{_7}{_8}{_9}{_10}{_11}{_12}{_13}{_14}{_15}{_16}{_17}{_18}{_19}{_20}{_21}{_22}{_23}{_24}{_25}{_26}{_27}{_28}{_29}{_30}{_31}{_32}{_33}{_34}{_35}{_36}{_37}{_38}{_39}{_40}</p>
+<slot dummy={dummy}></slot>
+<button on:click={increment}></button>
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot-4/_config.js b/test/runtime/samples/bitmask-overflow-slot-4/_config.js
new file mode 100644
index 000000000000..cdaa5de77fa6
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-4/_config.js
@@ -0,0 +1,41 @@
+export default {
+	html: `
+		<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+		<p>0</p>
+		<p>0</p>
+		<button></button>
+	`,
+
+	async test({ assert, component, target, window }) {
+		// change from inside
+		const button = target.querySelector('button');
+		await button.dispatchEvent(new window.MouseEvent('click'));
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+			<p>0</p>
+			<p>1</p>
+			<button></button>
+		`);
+
+		// change from outside
+		component._0 = 'a';
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+			<p>a</p>
+			<p>1</p>
+			<button></button>
+		`);
+
+		// change from outside through props
+		component._40 = 'b';
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39b</p>
+			<p>a</p>
+			<p>1</p>
+			<button></button>
+		`);
+	}
+};
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot-4/main.svelte b/test/runtime/samples/bitmask-overflow-slot-4/main.svelte
new file mode 100644
index 000000000000..7e02487a305d
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-4/main.svelte
@@ -0,0 +1,12 @@
+<script>
+	import Echo from './Echo.svelte';
+
+	export let _0 = 0, _40;
+	
+</script>
+
+<Echo _40={_40} let:dummy>
+	<p>{_0}</p>
+	<p>{dummy}</p>
+</Echo>
+
diff --git a/test/runtime/samples/bitmask-overflow-slot-5/Echo.svelte b/test/runtime/samples/bitmask-overflow-slot-5/Echo.svelte
new file mode 100644
index 000000000000..dddb3f76428f
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-5/Echo.svelte
@@ -0,0 +1,13 @@
+<script>
+	export let _0 = '_0', _1 = '_1', _2 = '_2', _3 = '_3', _4 = '_4', _5 = '_5', _6 = '_6', _7 = '_7', _8 = '_8', _9 = '_9', _10 = '_10', _11 = '_11', _12 = '_12', _13 = '_13', _14 = '_14', _15 = '_15', _16 = '_16', _17 = '_17', _18 = '_18', _19 = '_19', _20 = '_20', _21 = '_21', _22 = '_22', _23 = '_23', _24 = '_24', _25 = '_25', _26 = '_26', _27 = '_27', _28 = '_28', _29 = '_29', _30 = '_30', _31 = '_31', _32 = '_32', _33 = '_33', _34 = '_34', _35 = '_35', _36 = '_36', _37 = '_37', _38 = '_38', _39 = '_39', _40 = '_40';	
+	export let b = 'b';
+	let dummy = 0;
+	function increment () {
+		dummy = 1;
+	}
+</script>
+
+<p>{_0}{_1}{_2}{_3}{_4}{_5}{_6}{_7}{_8}{_9}{_10}{_11}{_12}{_13}{_14}{_15}{_16}{_17}{_18}{_19}{_20}{_21}{_22}{_23}{_24}{_25}{_26}{_27}{_28}{_29}{_30}{_31}{_32}{_33}{_34}{_35}{_36}{_37}{_38}{_39}{_40}</p>
+<p>{b}</p>
+<slot dummy={dummy}></slot>
+<button on:click={increment}></button>
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot-5/_config.js b/test/runtime/samples/bitmask-overflow-slot-5/_config.js
new file mode 100644
index 000000000000..7dedb8f7ebde
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-5/_config.js
@@ -0,0 +1,49 @@
+export default {
+	html: `
+		<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+		<p>b</p>
+		<p>-0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18-19-20-21-22-23-24-25-26-27-28-29-30-31-32-33-34-35-36-37-38-39-40</p>
+		<p>0</p>
+		<p>0</p>
+		<button></button>
+	`,
+
+	async test({ assert, component, target, window }) {
+		// change from inside
+		const button = target.querySelector('button');
+		await button.dispatchEvent(new window.MouseEvent('click'));
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+			<p>b</p>
+			<p>-0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18-19-20-21-22-23-24-25-26-27-28-29-30-31-32-33-34-35-36-37-38-39-40</p>
+			<p>0</p>
+			<p>1</p>
+			<button></button>
+		`);
+
+		// change from outside
+		component.a = 'AA';
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+			<p>b</p>
+			<p>-0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18-19-20-21-22-23-24-25-26-27-28-29-30-31-32-33-34-35-36-37-38-39-40</p>
+			<p>AA</p>
+			<p>1</p>
+			<button></button>
+		`);
+
+		// change from outside through props
+		component.b = 'BB';
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>_0_1_2_3_4_5_6_7_8_9_10_11_12_13_14_15_16_17_18_19_20_21_22_23_24_25_26_27_28_29_30_31_32_33_34_35_36_37_38_39_40</p>
+			<p>BB</p>
+			<p>-0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16-17-18-19-20-21-22-23-24-25-26-27-28-29-30-31-32-33-34-35-36-37-38-39-40</p>
+			<p>AA</p>
+			<p>1</p>
+			<button></button>
+		`);
+	}
+};
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot-5/main.svelte b/test/runtime/samples/bitmask-overflow-slot-5/main.svelte
new file mode 100644
index 000000000000..b17d6f7baef8
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot-5/main.svelte
@@ -0,0 +1,13 @@
+<script>
+	import Echo from './Echo.svelte';
+	export let _0 = '-0', _1 = '-1', _2 = '-2', _3 = '-3', _4 = '-4', _5 = '-5', _6 = '-6', _7 = '-7', _8 = '-8', _9 = '-9', _10 = '-10', _11 = '-11', _12 = '-12', _13 = '-13', _14 = '-14', _15 = '-15', _16 = '-16', _17 = '-17', _18 = '-18', _19 = '-19', _20 = '-20', _21 = '-21', _22 = '-22', _23 = '-23', _24 = '-24', _25 = '-25', _26 = '-26', _27 = '-27', _28 = '-28', _29 = '-29', _30 = '-30', _31 = '-31', _32 = '-32', _33 = '-33', _34 = '-34', _35 = '-35', _36 = '-36', _37 = '-37', _38 = '-38', _39 = '-39', _40 = '-40';	
+	export let a = 0, b;
+	
+</script>
+
+<Echo b={b} let:dummy>
+	<p>{_0}{_1}{_2}{_3}{_4}{_5}{_6}{_7}{_8}{_9}{_10}{_11}{_12}{_13}{_14}{_15}{_16}{_17}{_18}{_19}{_20}{_21}{_22}{_23}{_24}{_25}{_26}{_27}{_28}{_29}{_30}{_31}{_32}{_33}{_34}{_35}{_36}{_37}{_38}{_39}{_40}</p>
+	<p>{a}</p>
+	<p>{dummy}</p>
+</Echo>
+