diff --git a/data_safe_haven/functions/typer_validators.py b/data_safe_haven/functions/typer_validators.py
index 30d6227249..df72cd149c 100644
--- a/data_safe_haven/functions/typer_validators.py
+++ b/data_safe_haven/functions/typer_validators.py
@@ -16,6 +16,7 @@ def typer_validator_factory(validator: Callable[[Any], Any]) -> Callable[[Any],
     def typer_validator(x: Any) -> Any:
         try:
             validator(x)
+            return x
         except ValueError as exc:
             raise BadParameter(str(exc)) from exc
 
diff --git a/tests_/functions/test_typer_validators.py b/tests_/functions/test_typer_validators.py
new file mode 100644
index 0000000000..0c9c02ff90
--- /dev/null
+++ b/tests_/functions/test_typer_validators.py
@@ -0,0 +1,28 @@
+import pytest
+from typer import BadParameter
+
+from data_safe_haven.functions.typer_validators import typer_validate_aad_guid
+
+
+class TestTyperValidateAadGuid:
+    @pytest.mark.parametrize(
+        "guid",
+        [
+            "d5c5c439-1115-4cb6-ab50-b8e547b6c8dd",
+            "10de18e7-b238-6f1e-a4ad-772708929203",
+        ]
+    )
+    def test_typer_validate_aad_guid(self, guid):
+        assert typer_validate_aad_guid(guid) == guid
+
+    @pytest.mark.parametrize(
+        "guid",
+        [
+            "10de18e7_b238_6f1e_a4ad_772708929203",
+            "not a guid",
+        ]
+    )
+    def test_typer_validate_aad_guid_fail(self, guid):
+        with pytest.raises(BadParameter) as exc:
+            typer_validate_aad_guid(guid)
+            assert "Expected GUID" in exc
diff --git a/tests_/functions/test_validators.py b/tests_/functions/test_validators.py
new file mode 100644
index 0000000000..bcf41ca263
--- /dev/null
+++ b/tests_/functions/test_validators.py
@@ -0,0 +1,27 @@
+import pytest
+
+from data_safe_haven.functions.validators import validate_aad_guid
+
+
+class TestValidateAadGuid:
+    @pytest.mark.parametrize(
+        "guid",
+        [
+            "d5c5c439-1115-4cb6-ab50-b8e547b6c8dd",
+            "10de18e7-b238-6f1e-a4ad-772708929203",
+        ]
+    )
+    def test_validate_aad_guid(self, guid):
+        assert validate_aad_guid(guid) == guid
+
+    @pytest.mark.parametrize(
+        "guid",
+        [
+            "10de18e7_b238_6f1e_a4ad_772708929203",
+            "not a guid",
+        ]
+    )
+    def test_validate_aad_guid_fail(self, guid):
+        with pytest.raises(ValueError) as exc:
+            validate_aad_guid(guid)
+            assert "Expected GUID" in exc