diff --git a/pkgs/applications/misc/auto-multiple-choice/default.nix b/pkgs/applications/misc/auto-multiple-choice/default.nix
index 900653633a4..8617e345423 100644
--- a/pkgs/applications/misc/auto-multiple-choice/default.nix
+++ b/pkgs/applications/misc/auto-multiple-choice/default.nix
@@ -42,8 +42,14 @@ stdenv.mkDerivation rec {
     # Relative paths.
     "BINDIR=/bin"
     "PERLDIR=/share/perl5"
-    "MODSDIR=/nonexistent" # AMC will test for that dir before
-    # defaulting to the "portable" strategy, so this test *must* fail.
+    "MODSDIR=/lib" # At runtime, AMC will test for that dir before
+    # defaulting to the "portable" strategy we use, so this test
+    # *must* fail.  *But* this variable cannot be set to anything but
+    # "/lib" , because that name is hardcoded in the main executable
+    # and this variable controls both both the path AMC will check at
+    # runtime, AND the path where the actual modules will be stored at
+    # build-time.  This has been reported upstream as
+    # https://project.auto-multiple-choice.net/issues/872
     "TEXDIR=/tex/latex/" # what texlive.combine expects
     "TEXDOCDIR=/share/doc/texmf/" # TODO where to put this?
     "MAN1DIR=/share/man/man1"