diff --git a/ui/events/keycodes/keyboard_code_conversion_x.cc b/ui/events/keycodes/keyboard_code_conversion_x.cc
index 4e0882b..daba8115 100644
--- a/ui/events/keycodes/keyboard_code_conversion_x.cc
+++ b/ui/events/keycodes/keyboard_code_conversion_x.cc
@@ -545,8 +545,11 @@
   }
 
   keycode = KeyboardCodeFromXKeysym(keysym);
-  if (keycode == VKEY_UNKNOWN)
+  if (keycode == VKEY_UNKNOWN && !IsModifierKey(keysym)) {
+    // Modifier keys should not fall back to the hardware-keycode-based US
+    // layout.  See crbug.com/402320
     keycode = DefaultKeyboardCodeFromHardwareKeycode(xkey->keycode);
+  }
 
   return keycode;
 }