diff --git a/tierkreis/tierkreis/builder.py b/tierkreis/tierkreis/builder.py
index 7d3c7398..d3a377f6 100644
--- a/tierkreis/tierkreis/builder.py
+++ b/tierkreis/tierkreis/builder.py
@@ -15,7 +15,7 @@
     runtime_checkable,
 )
 
-from tierkreis.controller.data.core import EmptyModel
+from tierkreis.controller.data.core import EmptyModel, RestrictedNamedTuple
 from tierkreis.controller.data.graph import GraphData, ValueRef, reindex_inputs
 from tierkreis.controller.data.models import (
     TKR,
@@ -441,7 +441,7 @@ def map(
 
         if not isclass(body.outputs_type) or not issubclass(
             body.outputs_type,
-            TNamedModel,
+            RestrictedNamedTuple,  # Cannot check TNamedModel
         ):
             out = self._fold_list(out)
 
diff --git a/tierkreis/tierkreis/controller/data/models.py b/tierkreis/tierkreis/controller/data/models.py
index df117053..571c78e5 100644
--- a/tierkreis/tierkreis/controller/data/models.py
+++ b/tierkreis/tierkreis/controller/data/models.py
@@ -13,7 +13,6 @@
     get_args,
     get_origin,
     overload,
-    runtime_checkable,
 )
 
 from typing_extensions import TypeIs
@@ -29,7 +28,7 @@
 TKR_PORTMAPPING_FLAG = "__tkr_portmapping__"
 
 
-@runtime_checkable
+# @runtime_checkable # ALAN no, as we cannot distinguish between this and TNamedModel
 class PNamedModel(RestrictedNamedTuple[PType], Protocol):
     """A struct whose members are restricted to being PTypes.
 
@@ -56,7 +55,7 @@ def value_ref(self) -> ValueRef:
         return (self.node_index, self.port_id)
 
 
-@runtime_checkable
+# @runtime_checkable # ALAN no, as we cannot distinguish between this and PNamedModel
 class TNamedModel(RestrictedNamedTuple[TKR[PType] | None], Protocol):
     """A struct whose members are restricted to being references to PTypes.
 
@@ -93,7 +92,9 @@ def is_tnamedmodel(o) -> TypeIs[type[TNamedModel]]:  # noqa: ANN001 inherited fr
     origin = get_origin(o)
     if origin is not None:
         return is_tnamedmodel(origin)
-    return isclass(o) and issubclass(o, TNamedModel)
+    # We cannot check whether fields are TNamedModel-compliant at runtime.
+    # So this overapproximates.
+    return isclass(o) and issubclass(o, RestrictedNamedTuple)
 
 
 def dict_from_pmodel(pmodel: PModel) -> dict[PortID, PType]:
@@ -112,7 +113,8 @@ def annotations_from_pmodel(pmodel: type) -> dict[PortID, Any]:
 
 
 def dict_from_tmodel(tmodel: TModel) -> dict[PortID, ValueRef]:
-    if isinstance(tmodel, TNamedModel):
+    # We can't do the ideal: if isinstance(tmodel, TNamedModel):
+    if not isinstance(tmodel, TKR):
         return {k: (v.node_index, v.port_id) for k, v in tmodel._asdict().items() if v}
 
     return {"value": (tmodel.node_index, tmodel.port_id)}
diff --git a/tierkreis/tierkreis/controller/data/types.py b/tierkreis/tierkreis/controller/data/types.py
index 33ff544f..19563e61 100644
--- a/tierkreis/tierkreis/controller/data/types.py
+++ b/tierkreis/tierkreis/controller/data/types.py
@@ -111,7 +111,7 @@ def from_list(cls, arg: list, /) -> "Self":
 logger = logging.getLogger(__name__)
 
 
-@runtime_checkable
+# @runtime_checkable # ALAN no, as we cannot distinguish between this and TNamedModel
 class Struct(RestrictedNamedTuple[JsonType], Protocol):
     """Supertype for structs, which are named tuples with JSON-serialisable fields."""