diff --git a/qs/Makefile b/qs/Makefile
index 94a878849..30b9e65aa 100644
--- a/qs/Makefile
+++ b/qs/Makefile
@@ -4,13 +4,23 @@ OTHERFLAGS += --ext pulse:extract_ocaml_bare
 
 DEPFLAGS += --extract '-*'
 DEPFLAGS += --extract +Quicksort
-DEPFLAGS += --extract +Pulse.Lib.Dv
+# DEPFLAGS += --extract +Pulse.Lib.Dv
 # DEPFLAGS += --extract +Pulse.Lib.Task
 # DEPFLAGS += --extract +Pulse.Class.Duplicable
 # DEPFLAGS += --extract +FStar.Dyn # wrong. it's int FStarC_Dyn.ml
 
 include fstar.mk
 
+# This makefile only does something if a 5.1.1 OPAM switch is available.
+HAVE_511 != opam env --switch=5.1.1 2>/dev/null # only outputs to stdout on success
+
+ifeq ($(HAVE_511),)
+_ != echo "OPAM switch 5.1.1 does not seem installed, not doing anything" > /dev/tty
+.PHONY: all test
+all:
+test:
+else
+
 .PHONY: all
 all: dune
 
@@ -24,3 +34,4 @@ test: run
 .PHONY: run
 run: dune
 	./dune/_build/default/driver.exe
+endif
diff --git a/qs/README.md b/qs/README.md
index 9edde3b76..c99c4d34f 100644
--- a/qs/README.md
+++ b/qs/README.md
@@ -7,3 +7,6 @@ opam switch create 5.1.1
 opam install --switch=5.1.1 domainslib
 make
 ```
+
+Note: the Makefile here will not do anything if a 5.1.1 switch
+is not installed. Our CI does not test this directory.
diff --git a/qs/dune/Pulse_Lib_Dv.ml b/qs/dune/Pulse_Lib_Dv.ml
new file mode 100644
index 000000000..add7343ec
--- /dev/null
+++ b/qs/dune/Pulse_Lib_Dv.ml
@@ -0,0 +1,8 @@
+(* This is a slightly edited version of lib/pulse/lib/ml/Pulse_Lib_Dv.ml, to make it work in OCaml 5 (just remove the val) *)
+
+let while_ cond body =
+  while (cond ()) do
+    body ()
+  done
+
+let rec unreachable () : 'a = unreachable ()