diff --git a/src/runtime/internal/utils.ts b/src/runtime/internal/utils.ts
index fb2554ad78..b844f1dd4c 100644
--- a/src/runtime/internal/utils.ts
+++ b/src/runtime/internal/utils.ts
@@ -80,9 +80,17 @@ export function get_slot_changes(definition, $$scope, dirty, fn) {
 	if (definition[2] && fn) {
 		const lets = definition[2](fn(dirty));
 
-		return typeof $$scope.dirty === 'object'
-			? $$scope.dirty.map((n, i) => n | lets[i])
-			: $$scope.dirty | lets;
+		if (typeof $$scope.dirty === 'object') {
+			const merged = [];
+			const len = Math.max($$scope.dirty.length, lets.length);
+			for (let i = 0; i < len; i += 1) {
+				merged[i] = $$scope.dirty[i] | lets[i];
+			}
+
+			return merged;
+		}
+
+		return $$scope.dirty | lets;
 	}
 
 	return $$scope.dirty;
diff --git a/test/runtime/samples/bitmask-overflow-slot/Echo.svelte b/test/runtime/samples/bitmask-overflow-slot/Echo.svelte
new file mode 100644
index 0000000000..28eaa54060
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot/Echo.svelte
@@ -0,0 +1,5 @@
+<script>
+	export let dummy;
+</script>
+
+<slot dummy={dummy}></slot>
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot/_config.js b/test/runtime/samples/bitmask-overflow-slot/_config.js
new file mode 100644
index 0000000000..9b24d5541f
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot/_config.js
@@ -0,0 +1,124 @@
+export default {
+	html: `
+		<p>0</p>
+		<p>1</p>
+		<p>2</p>
+		<p>3</p>
+		<p>4</p>
+		<p>5</p>
+		<p>6</p>
+		<p>7</p>
+		<p>8</p>
+		<p>9</p>
+		<p>10</p>
+		<p>11</p>
+		<p>12</p>
+		<p>13</p>
+		<p>14</p>
+		<p>15</p>
+		<p>16</p>
+		<p>17</p>
+		<p>18</p>
+		<p>19</p>
+		<p>20</p>
+		<p>21</p>
+		<p>22</p>
+		<p>23</p>
+		<p>24</p>
+		<p>25</p>
+		<p>26</p>
+		<p>27</p>
+		<p>28</p>
+		<p>29</p>
+		<p>30</p>
+		<p>31</p>
+		<p>32</p>
+		<p>33</p>
+		<p>34</p>
+		<p>35</p>
+		<p>36</p>
+		<p>37</p>
+		<p>38</p>
+		<p>39</p>
+		<p>40</p>
+		<p>5:36</p>
+		<p>6:37</p>
+		<p>38</p>
+		<p>0</p>
+	`,
+
+	test({ assert, component, target }) {
+		component.reads = {};
+
+		component._0 = 'a';
+		component._30 = 'b';
+		component._31 = 'c';
+		component._32 = 'd';
+		component._40 = 'e';
+
+		component._5 = 'f';
+		component._6 = 'g';
+		component._36 = 'h';
+		component._37 = 'i';
+
+		assert.htmlEqual(target.innerHTML, `
+			<p>a</p>
+			<p>1</p>
+			<p>2</p>
+			<p>3</p>
+			<p>4</p>
+			<p>f</p>
+			<p>g</p>
+			<p>7</p>
+			<p>8</p>
+			<p>9</p>
+			<p>10</p>
+			<p>11</p>
+			<p>12</p>
+			<p>13</p>
+			<p>14</p>
+			<p>15</p>
+			<p>16</p>
+			<p>17</p>
+			<p>18</p>
+			<p>19</p>
+			<p>20</p>
+			<p>21</p>
+			<p>22</p>
+			<p>23</p>
+			<p>24</p>
+			<p>25</p>
+			<p>26</p>
+			<p>27</p>
+			<p>28</p>
+			<p>29</p>
+			<p>b</p>
+			<p>c</p>
+			<p>d</p>
+			<p>33</p>
+			<p>34</p>
+			<p>35</p>
+			<p>h</p>
+			<p>i</p>
+			<p>38</p>
+			<p>39</p>
+			<p>e</p>
+			<p>f:h</p>
+			<p>g:i</p>
+			<p>38</p>
+			<p>a</p>
+		`);
+
+		assert.deepEqual(component.reads, {
+			_0: 1,
+			_5: 3,
+			_6: 3,
+			_30: 1,
+			_31: 1,
+			_32: 1,
+			_36: 3,
+			_37: 3,
+			_40: 1
+		});
+	}
+};
\ No newline at end of file
diff --git a/test/runtime/samples/bitmask-overflow-slot/main.svelte b/test/runtime/samples/bitmask-overflow-slot/main.svelte
new file mode 100644
index 0000000000..89e60ce4b9
--- /dev/null
+++ b/test/runtime/samples/bitmask-overflow-slot/main.svelte
@@ -0,0 +1,107 @@
+<script>
+	import Echo from './Echo.svelte';
+
+	export let reads = {};
+
+	export let _0 = '0';
+	export let _1 = '1';
+	export let _2 = '2';
+	export let _3 = '3';
+	export let _4 = '4';
+	export let _5 = '5';
+	export let _6 = '6';
+	export let _7 = '7';
+	export let _8 = '8';
+	export let _9 = '9';
+	export let _10 = '10';
+	export let _11 = '11';
+	export let _12 = '12';
+	export let _13 = '13';
+	export let _14 = '14';
+	export let _15 = '15';
+	export let _16 = '16';
+	export let _17 = '17';
+	export let _18 = '18';
+	export let _19 = '19';
+	export let _20 = '20';
+	export let _21 = '21';
+	export let _22 = '22';
+	export let _23 = '23';
+	export let _24 = '24';
+	export let _25 = '25';
+	export let _26 = '26';
+	export let _27 = '27';
+	export let _28 = '28';
+	export let _29 = '29';
+	export let _30 = '30';
+	export let _31 = '31';
+	export let _32 = '32';
+	export let _33 = '33';
+	export let _34 = '34';
+	export let _35 = '35';
+	export let _36 = '36';
+	export let _37 = '37';
+	export let _38 = '38';
+	export let _39 = '39';
+	export let _40 = '40';
+
+	$: foo = read(_6, '_6') + ':' + read(_37, '_37');
+	$: bar = read(_38, '_38');
+
+	const read = (value, label) => {
+		if (!reads[label]) reads[label] = 0;
+		reads[label] += 1;
+
+		return value;
+	};
+</script>
+
+<Echo dummy={_0} let:dummy>
+	<p>{read(_0, '_0')}</p>
+	<p>{read(_1, '_1')}</p>
+	<p>{read(_2, '_2')}</p>
+	<p>{read(_3, '_3')}</p>
+	<p>{read(_4, '_4')}</p>
+	<p>{read(_5, '_5')}</p>
+	<p>{read(_6, '_6')}</p>
+	<p>{read(_7, '_7')}</p>
+	<p>{read(_8, '_8')}</p>
+	<p>{read(_9, '_9')}</p>
+	<p>{read(_10, '_10')}</p>
+	<p>{read(_11, '_11')}</p>
+	<p>{read(_12, '_12')}</p>
+	<p>{read(_13, '_13')}</p>
+	<p>{read(_14, '_14')}</p>
+	<p>{read(_15, '_15')}</p>
+	<p>{read(_16, '_16')}</p>
+	<p>{read(_17, '_17')}</p>
+	<p>{read(_18, '_18')}</p>
+	<p>{read(_19, '_19')}</p>
+	<p>{read(_20, '_20')}</p>
+	<p>{read(_21, '_21')}</p>
+	<p>{read(_22, '_22')}</p>
+	<p>{read(_23, '_23')}</p>
+	<p>{read(_24, '_24')}</p>
+	<p>{read(_25, '_25')}</p>
+	<p>{read(_26, '_26')}</p>
+	<p>{read(_27, '_27')}</p>
+	<p>{read(_28, '_28')}</p>
+	<p>{read(_29, '_29')}</p>
+	<p>{read(_30, '_30')}</p>
+	<p>{read(_31, '_31')}</p>
+	<p>{read(_32, '_32')}</p>
+	<p>{read(_33, '_33')}</p>
+	<p>{read(_34, '_34')}</p>
+	<p>{read(_35, '_35')}</p>
+	<p>{read(_36, '_36')}</p>
+	<p>{read(_37, '_37')}</p>
+	<p>{read(_38, '_38')}</p>
+	<p>{read(_39, '_39')}</p>
+	<p>{read(_40, '_40')}</p>
+
+	<p>{read(_5, '_5') + ':' + read(_36, '_36')}</p>
+	<p>{foo}</p>
+	<p>{bar}</p>
+
+	<p>{dummy}</p>
+</Echo>
\ No newline at end of file