diff --git a/rocrate/rocrate.py b/rocrate/rocrate.py
index 3e75cd53..2944bd5d 100644
--- a/rocrate/rocrate.py
+++ b/rocrate/rocrate.py
@@ -693,10 +693,11 @@ def add_workflow(
         ))
         if isinstance(lang, ComputerLanguage):
             assert lang.crate is self
+            assert self.get(lang.id) is lang
         else:
             lang = get_lang(self, lang, version=lang_version)
             self.add(lang)
-        lang_str = lang.id.rsplit("#", 1)[1]
+        lang_str = lang.id.rsplit("#", 1)[-1]
         workflow.lang = lang
         if main:
             self.mainEntity = workflow
diff --git a/test/test_workflow_ro_crate.py b/test/test_workflow_ro_crate.py
index e41ad6a2..b2b09db2 100644
--- a/test/test_workflow_ro_crate.py
+++ b/test/test_workflow_ro_crate.py
@@ -24,6 +24,7 @@
 
 import pytest
 
+from rocrate.model import ComputerLanguage
 from rocrate.rocrate import ROCrate, make_workflow_rocrate
 try:
     import galaxy2cwl  # noqa: F401
@@ -158,3 +159,23 @@ def test_cwl_lang_version(test_data_dir, lang_version):
     else:
         assert lang_id == "https://w3id.org/cwl/v1.2/"
         assert lang["version"] == "v1.2"
+
+
+def test_custom_lang(tmpdir):
+    wf_id = 'custom_wf.foo'
+    wf_path = tmpdir / wf_id
+    wf_path.write_text("Step1: foo\nStep2: bar\n")
+    crate = ROCrate()
+    lang_id = "https://example.org/lang/foo"
+    lang = ComputerLanguage(
+        crate,
+        identifier=lang_id,
+        properties={
+            "name": "Foo language",
+            "identifier": {"@id": lang_id},
+            "url": {"@id": lang_id}
+        },
+    )
+    crate.add(lang)
+    workflow = crate.add_workflow(wf_path, wf_id, lang=lang)
+    assert workflow["programmingLanguage"].id == lang_id