diff --git a/test/tw-1637.t b/test/tw-1637.t
index 05ab1f413..ed852d392 100755
--- a/test/tw-1637.t
+++ b/test/tw-1637.t
@@ -2,6 +2,8 @@
 . bash_tap_tw.sh
 
 # This problems only appears for "modify", not for "add"
+# The 'proj:mod' is interpreted as 'proj:modified', then DOM lookup substitutes
+# the 'modified' date.
 task add a
 task 1 mod proj:mod
 task 1 _project | grep mod