diff --git a/unittests/test_crawler.py b/unittests/test_crawler.py
index 677fd1e5e83bc97e27b11240b972017cbf762289..920728c418479cd594e894e990d5f15693be589d 100644
--- a/unittests/test_crawler.py
+++ b/unittests/test_crawler.py
@@ -441,6 +441,8 @@ a: ([b1, b2])
     prop_ident = db.Property("prop_ident", datatype=db.INTEGER)
     prop_other = db.Property("prop_ident", datatype=db.INTEGER)
     rt_c = db.RecordType("C").add_property(prop_other)
+    # Somehow it is necessary that `B` has a reference property.  Dunno if C must have an
+    # identifiable as well.
     rt_b = db.RecordType("B").add_property(prop_ident).add_property("C")
     rt_a = db.RecordType("A").add_property(prop_ident).add_property("LIST<B>")