diff --git a/master_changes.md b/master_changes.md
index 5208994bd3f..b3798a7fc80 100644
--- a/master_changes.md
+++ b/master_changes.md
@@ -27,6 +27,7 @@ users)
 ## Install
   * [BUG] On install driven by `.install` file, track intermediate directories too, in order to have them suppressed at package removal [#5691 @rjbou - fix #5688]
   * [BUG] With `--assume-built`, resolve variables in depends filter according switch & global environment, not only depends predefined variables [#570 @rjbou - fix #5698]
+  * [BUG] Handle undefined variables defaults to false in dependencies formula resolution for assume built [#5701 rjbou]
 
 ## Remove
 
@@ -119,6 +120,7 @@ users)
   * dot-install: generalise inner script & use less generic filenames [#5691 @rjbou]
   * dot-install: add a test for removal of non specified in .install empty directories [#5701 @rjbou]
   * Add test in assume-built for depends with switch variable filters [#5700 @rjbou]
+  * Add undefined variable handling in assume built test [#5701 @rjbou]
 
 ### Engine
   * With real path resolved for all opam temp dir, remove `/private` from mac temp dir regexp [#5654 @rjbou]
diff --git a/src/client/opamClient.ml b/src/client/opamClient.ml
index cd2863966ad..7d44490cf71 100644
--- a/src/client/opamClient.ml
+++ b/src/client/opamClient.ml
@@ -1096,7 +1096,7 @@ let check_installed ~build ~post t atoms =
             let cnf_formula =
               OpamSwitchState.opam t pkg
               |> OpamFile.OPAM.depends
-              |> OpamFilter.filter_formula (env pkg)
+              |> OpamFilter.filter_formula ~default:false (env pkg)
               |> OpamFormula.to_cnf
             in
             let missing_conj =
diff --git a/tests/reftests/assume-built.test b/tests/reftests/assume-built.test
index df81624afae..ee021b5f611 100644
--- a/tests/reftests/assume-built.test
+++ b/tests/reftests/assume-built.test
@@ -225,3 +225,49 @@ The following actions will be performed:
 -> installed ongoing.1.0
 Done.
 ### test -f ./ongoing/installed
+### : assume built & undefined opam file variables :
+### OPAMSTRICT=0
+### opam unpin ongoing
+Ok, ongoing is no longer pinned to git+file://${BASEDIR}/ongoing#master (version 1.0)
+The following actions will be performed:
+=== remove 1 package
+  - remove ongoing 1.0
+
+<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
+-> removed   ongoing.1.0
+Done.
+### <pin:ongoing/ongoing.opam>
+opam-version: "2.0"
+depends: [
+  "qux"
+  "foo" { not-defined }
+]
+build: [
+  [ "test" "-f" "ongoing.txt" ]
+  [ "sh" "-c" "cat ongoing.txt > out" ]
+]
+install: [
+  [ "test" "-f" "out" ]
+  [ "test" "-f" "out2" ] { not-defined }
+  [ "touch" "installed"]
+]
+### git -C ongoing clean -fdqx
+### opam pin ./ongoing -n
+Package ongoing does not exist, create as a NEW package? [y/n] y
+ongoing is now pinned to git+file://${BASEDIR}/ongoing#master (version dev)
+### touch ongoing/out
+### opam install ongoing --assume-built --with-test
+
+<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
+[NOTE] Ignoring uncommitted changes in ${BASEDIR}/ongoing (`--working-dir' not specified or specified with no argument).
+[ongoing.dev] synchronised (no changes)
+
+The following actions will be performed:
+=== install 1 package
+  - install ongoing dev (pinned)
+
+<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
+-> retrieved ongoing.dev  (no changes)
+-> installed ongoing.dev
+Done.
+### test -f ./ongoing/installed