diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index dbb313b06ff..029f7560da0 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -1955,6 +1955,13 @@ EOF
 
 This is a bug in HPUX; HPUX patch PHKL_16260 is said to fix it.
 
+*** HP-UX 11.31 cc: Emacs does not build.
+
+HP-UX 11.31 cc has bugs in its implementation of flexible array
+members, a C99 feature that Emacs relies on.  To work around the
+problem, install GCC and use it to build Emacs.  For details, see:
+http://lists.gnu.org/archive/html/bug-gnulib/2017-03/msg00066.html
+
 ** AIX
 
 *** AIX: Trouble using ptys.