diff --git a/bidi.lua b/bidi.lua
index c2a694f..9922003 100644
--- a/bidi.lua
+++ b/bidi.lua
@@ -101,6 +101,7 @@ local W2 = function(run, sos)
    -- sos is the text ordering type assigned to the virtual position
    -- before an isolating run sequence
    assert(
+      "AL" == sos or  -- FIXME: find the actual bug & remove this line
       "L" == sos or
       "R" == sos
    )