diff --git a/src/hotkey.ts b/src/hotkey.ts
index 91a9a0f..3ca6e3d 100644
--- a/src/hotkey.ts
+++ b/src/hotkey.ts
@@ -76,7 +76,7 @@ const modifierKeyNames: string[] = ['Control', 'Alt', 'Meta', 'Shift']
  *   platforms.
  * - Ensures modifiers are sorted in a consistent order
  * @param hotkey a hotkey string
- * @param platform NOTE: this param is only intended to be used to mock `navigator.platform` in tests
+ * @param platform NOTE: this param is only intended to be used to mock `navigator.platform` in tests. `window.navigator.platform` is used by default.
  * @returns {string} normalized representation of the given hotkey string
  */
 export function normalizeHotkey(hotkey: string, platform?: string | undefined): NormalizedHotkeyString {
@@ -88,7 +88,7 @@ export function normalizeHotkey(hotkey: string, platform?: string | undefined):
 
 const matchApplePlatform = /Mac|iPod|iPhone|iPad/i
 
-function localizeMod(hotkey: string, platform?: string): string {
+function localizeMod(hotkey: string, platform: string): string {
   const ssrSafeWindow = typeof window === "undefined" ? undefined : window
   const safePlatform = ssrSafeWindow
     ? ssrSafeWindow.navigator.platform