diff --git a/src/runtime/internal/utils.ts b/src/runtime/internal/utils.ts
index 46df7aa0bc..928882552a 100644
--- a/src/runtime/internal/utils.ts
+++ b/src/runtime/internal/utils.ts
@@ -11,7 +11,7 @@ export function assign<T, S>(tar: T, src: S): T & S {
 }
 
 export function is_promise<T = any>(value: any): value is PromiseLike<T> {
-	return value && typeof value === 'object' && typeof value.then === 'function';
+	return value && typeof value === 'object' && is_function(value.then);
 }
 
 export function add_location(element, file, line, column, char) {
@@ -33,7 +33,7 @@ export function is_function(thing: any): thing is Function {
 }
 
 export function safe_not_equal(a, b) {
-	return a != a ? b == b : a !== b || ((a && typeof a === 'object') || typeof a === 'function');
+	return a != a ? b == b : a !== b || ((a && typeof a === 'object') || is_function(a));
 }
 
 export function not_equal(a, b) {
@@ -45,7 +45,7 @@ export function is_empty(obj) {
 }
 
 export function validate_store(store, name) {
-	if (store != null && typeof store.subscribe !== 'function') {
+	if (store != null && !is_function(store.subscribe)) {
 		throw new Error(`'${name}' is not a store with a 'subscribe' method`);
 	}
 }
diff --git a/src/runtime/motion/tweened.ts b/src/runtime/motion/tweened.ts
index 83434cdff6..8fa5ebaba6 100644
--- a/src/runtime/motion/tweened.ts
+++ b/src/runtime/motion/tweened.ts
@@ -1,5 +1,5 @@
 import { Readable, writable } from 'svelte/store';
-import { assign, loop, now, Task } from 'svelte/internal';
+import { assign, is_function, loop, now, Task } from 'svelte/internal';
 import { linear } from 'svelte/easing';
 import { is_date } from './utils';
 
@@ -111,7 +111,7 @@ export function tweened<T>(value?: T, defaults: Options<T> = {}): Tweened<T> {
 
 			if (!started) {
 				fn = interpolate(value, new_value);
-				if (typeof duration === 'function') duration = duration(value, new_value);
+				if (is_function(duration)) duration = duration(value, new_value);
 				started = true;
 			}
 
diff --git a/src/runtime/transition/index.ts b/src/runtime/transition/index.ts
index 7e879cb941..dbddfcc874 100644
--- a/src/runtime/transition/index.ts
+++ b/src/runtime/transition/index.ts
@@ -186,7 +186,7 @@ export function draw(node: SVGElement & { getTotalLength(): number }, {
 		} else {
 			duration = len / speed;
 		}
-	} else if (typeof duration === 'function') {
+	} else if (is_function(duration)) {
 		duration = duration(len);
 	}