diff --git a/test/blackbox-tests/test-cases/explicit_js_mode/dune b/test/blackbox-tests/test-cases/explicit_js_mode/dune
index 3356f1be8fa5..3a8c851c3c03 100755
--- a/test/blackbox-tests/test-cases/explicit_js_mode/dune
+++ b/test/blackbox-tests/test-cases/explicit_js_mode/dune
@@ -1,6 +1,17 @@
 (rule (with-stdout-to foo.ml (echo "")))
 (rule (with-stdout-to bar.ml (echo "")))
 (rule (with-stdout-to cha.ml (echo "")))
+(rule (with-stdout-to zzz.ml (echo "")))
+(rule (with-stdout-to zzz2.ml (echo "")))
+
+(library
+ (name zzz)
+ (modes js)
+ (modules zzz))
+
+(library
+ (name zzz2)
+ (modules zzz2))
 
 (executable
  (name foo)
diff --git a/test/blackbox-tests/test-cases/explicit_js_mode/run.t b/test/blackbox-tests/test-cases/explicit_js_mode/run.t
index 961be68c667d..1983737dc357 100755
--- a/test/blackbox-tests/test-cases/explicit_js_mode/run.t
+++ b/test/blackbox-tests/test-cases/explicit_js_mode/run.t
@@ -11,6 +11,17 @@
    js_of_ocaml .js/stdlib/stdlib.cma.js
      jsoo_link bar.bc.js
 
+  $ dune clean
+  $ dune build --display short zzz.cma
+      ocamldep .zzz.objs/zzz.ml.d
+        ocamlc .zzz.objs/byte/zzz.{cmi,cmo,cmt}
+        ocamlc zzz.cma
+  $ dune build --display short _build/default/.zzz.objs/zzz.cma.js
+   js_of_ocaml .zzz.objs/zzz.cma.js
+  $ dune build --display short _build/default/.zzz2.objs/zzz2.cma.js
+  Don't know how to build _build/default/.zzz2.objs/zzz2.cma.js
+  [1]
+
   $ dune clean
   $ dune build --display short @all
       ocamldep .bar.eobjs/bar.ml.d
@@ -22,6 +33,12 @@
       ocamldep .foo.eobjs/foo.ml.d
         ocamlc .foo.eobjs/byte/foo.{cmi,cmo,cmt}
         ocamlc foo.bc
+      ocamldep .zzz.objs/zzz.ml.d
+        ocamlc .zzz.objs/byte/zzz.{cmi,cmo,cmt}
+        ocamlc zzz.cma
+      ocamldep .zzz2.objs/zzz2.ml.d
+        ocamlc .zzz2.objs/byte/zzz2.{cmi,cmo,cmt}
+        ocamlc zzz2.cma
         ocamlc .bar.eobjs/byte/bar.{cmi,cmo,cmt}
    js_of_ocaml .bar.eobjs/byte/bar.cmo.js
      jsoo_link bar.bc.js
@@ -29,4 +46,7 @@
       ocamlopt cha.exe
       ocamlopt .foo.eobjs/native/foo.{cmx,o}
       ocamlopt foo.exe
+      ocamlopt .zzz2.objs/native/zzz2.{cmx,o}
+      ocamlopt zzz2.{a,cmxa}
+      ocamlopt zzz2.cmxs
         ocamlc bar.bc