diff --git a/src/emulator.py b/src/emulator.py
index a01be7b..ec2fe81 100644
--- a/src/emulator.py
+++ b/src/emulator.py
@@ -421,6 +421,8 @@ def swipe(direction: str) -> None:
             debug.swipe_down()
         elif direction == "left":
             debug.swipe_left()
+        else:
+            raise ValueError(f"Unknown direction: {direction}")
 
 
 def assert_text_on_screen(debug: DebugLink, text: str) -> None: