diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 688aef7270..46b36ad25d 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2015-09-30  Doug Evans  <dje@google.com>
+
+	* dwarf2read.c (setup_type_unit_groups): Add comment.
+
 2015-09-30  Pedro Alves  <palves@redhat.com>
 
 	* linux-nat.c (linux_nat_always_non_stop_p): Always return 1.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 5b12342094..ddd4a4c932 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -9332,6 +9332,11 @@ setup_type_unit_groups (struct die_info *die, struct dwarf2_cu *cu)
     {
       struct compunit_symtab *cust = dwarf2_start_symtab (cu, "", NULL, 0);
 
+      /* Note: We don't assign tu_group->compunit_symtab yet because we're
+	 still initializing it, and our caller (a few levels up)
+	 process_full_type_unit still needs to know if this is the first
+	 time.  */
+
       tu_group->num_symtabs = lh->num_file_names;
       tu_group->symtabs = XNEWVEC (struct symtab *, lh->num_file_names);