diff --git a/src/ophyd_async/epics/signal/_common.py b/src/ophyd_async/epics/signal/_common.py
index 82070ef38e..34d344ba07 100644
--- a/src/ophyd_async/epics/signal/_common.py
+++ b/src/ophyd_async/epics/signal/_common.py
@@ -55,7 +55,7 @@ def get_supported_values(
                 f"which do not match {datatype}, which has {choices}."
             )
         return {x: datatype(x) if x else "_" for x in pv_choices}
-    elif datatype is None:
+    elif datatype is None or datatype is str:
         return {x: x or "_" for x in pv_choices}
 
     raise TypeError(
diff --git a/tests/epics/signal/test_common.py b/tests/epics/signal/test_common.py
index e45a78f031..7a16a59a51 100644
--- a/tests/epics/signal/test_common.py
+++ b/tests/epics/signal/test_common.py
@@ -35,8 +35,14 @@ class MyEnum(str, Enum):
         get_supported_values("", MyEnum, ("test",))
 
 
-def test_given_no_supplied_enum_then_returns_generated_choices_enum_with_pv_choices():
-    supported_vals = get_supported_values("", None, ("test",))
+@pytest.mark.parametrize(
+    "datatype",
+    [None, str],
+)
+def test_given_no_enum_or_string_then_returns_generated_choices_enum_with_pv_choices(
+    datatype,
+):
+    supported_vals = get_supported_values("", datatype, ("test",))
     assert len(supported_vals) == 1
     assert "test" in supported_vals
 
diff --git a/tests/epics/signal/test_signals.py b/tests/epics/signal/test_signals.py
index 61c0ca5d68..d0e418db2c 100644
--- a/tests/epics/signal/test_signals.py
+++ b/tests/epics/signal/test_signals.py
@@ -769,6 +769,19 @@ async def test_str_enum_returns_enum(ioc: IOC):
     assert val == "Bbb"
 
 
+async def test_str_datatype_in_mbbo(ioc: IOC):
+    backend = await ioc.make_backend(MyEnum, "enum")
+    pv_name = f"{ioc.protocol}://{PV_PREFIX}:{ioc.protocol}:enum"
+    sig = epics_signal_rw(str, pv_name)
+    datakey = await backend.get_datakey(sig.source)
+    assert datakey["choices"] == ["Aaa", "Bbb", "Ccc"]
+    await sig.connect()
+    description = await sig.describe()
+    assert description[""]["choices"] == ["Aaa", "Bbb", "Ccc"]
+    val = await sig.get_value()
+    assert val == "Bbb"
+
+
 async def test_runtime_enum_returns_str(ioc: IOC):
     await ioc.make_backend(MySubsetEnum, "enum")
     pv_name = f"{ioc.protocol}://{PV_PREFIX}:{ioc.protocol}:enum"