diff --git a/src/dune_pkg/opam_solver.ml b/src/dune_pkg/opam_solver.ml
index a2bfca37c10..123f205470e 100644
--- a/src/dune_pkg/opam_solver.ml
+++ b/src/dune_pkg/opam_solver.ml
@@ -707,7 +707,8 @@ let opam_package_to_lock_file_pkg
   let depexts =
     OpamFile.OPAM.depexts opam_file
     |> List.concat_map ~f:(fun (sys_pkgs, filter) ->
-      if OpamFilter.eval_to_bool (Solver_env.to_env solver_env) filter
+      let env = Solver_env.to_env solver_env in
+      if OpamFilter.eval_to_bool ~default:false env filter
       then OpamSysPkg.Set.to_list_map OpamSysPkg.to_string sys_pkgs
       else [])
   in
diff --git a/test/blackbox-tests/test-cases/pkg/depexts/unknown-variable.t b/test/blackbox-tests/test-cases/pkg/depexts/unknown-variable.t
index b2ab2ad1d85..ac07888b938 100644
--- a/test/blackbox-tests/test-cases/pkg/depexts/unknown-variable.t
+++ b/test/blackbox-tests/test-cases/pkg/depexts/unknown-variable.t
@@ -19,7 +19,7 @@ Make a project that uses the foo library:
 
 Locking should succeed and not include the "unzip" package
   $ dune pkg lock 2>&1 | head -n 1
-  Error: exception Failure("Undefined boolean filter value: foobar")
+  Solution for dune.lock:
 
   $ [ -e dune.lock/foo.pkg ] && cat dune.lock/foo.pkg
-  [1]
+  (version 0.0.1)