diff --git a/sources/SquirrelInputController.swift b/sources/SquirrelInputController.swift
index 939e673ca..b835f423f 100644
--- a/sources/SquirrelInputController.swift
+++ b/sources/SquirrelInputController.swift
@@ -31,6 +31,7 @@ final class SquirrelInputController: IMKInputController {
 
   // swiftlint:disable:next cyclomatic_complexity
   override func handle(_ event: NSEvent!, client sender: Any!) -> Bool {
+    guard let event = event else { return false }
     let modifiers = event.modifierFlags
     let changes = lastModifiers.symmetricDifference(modifiers)