diff --git a/src/spox/_fields.py b/src/spox/_fields.py
index 0b3c96d..ab6f114 100644
--- a/src/spox/_fields.py
+++ b/src/spox/_fields.py
@@ -8,10 +8,11 @@
 import warnings
 from collections.abc import Iterable, Iterator, Sequence
 from dataclasses import Field, dataclass
-from typing import Optional, Union, cast, get_type_hints
+from typing import Optional, Union, get_type_hints
 
 from ._attributes import Attr
 from ._exceptions import InferenceWarning
+from ._type_system import Optional as tOptional
 from ._value_prop import PropDict, PropValue
 from ._var import Var, _VarInfo
 
@@ -152,50 +153,17 @@ def fully_typed(self) -> bool:
             for var in self.get_var_infos().values()
         )
 
-
-@dataclass
-class BaseInputs(BaseVarInfos):
-    def vars(self, prop_values: PropDict) -> BaseVars:
-        vars_dict: dict[str, Union[Var, Optional[Var], Sequence[Var]]] = {}
-
-        for field in dataclasses.fields(self):
-            field_type = self._get_field_type(field)
-            field_value = getattr(self, field.name)
-
-            if field_type == VarFieldKind.SINGLE:
-                prop_value = cast(PropValue, prop_values.get(field.name, None))
-                vars_dict[field.name] = Var(field_value, prop_value)
-
-            elif field_type == VarFieldKind.OPTIONAL:
-                prop_value = cast(PropValue, prop_values.get(field.name, None))
-                vars_dict[field.name] = Var(field_value, prop_value)
-
-            elif field_type == VarFieldKind.VARIADIC:
-                vars = []
-
-                for i, var_info in enumerate(field_value):
-                    var_value = prop_values.get(f"{field.name}_{i}", None)
-                    assert isinstance(var_value, PropValue)
-                    vars.append(Var(var_info, var_value))
-
-                vars_dict[field.name] = vars
-
-        return BaseVars(vars_dict)
-
-
-@dataclass
-class BaseOutputs(BaseVarInfos):
-    def _propagate_vars(self, prop_values: Optional[PropDict] = None) -> BaseVars:
-        if prop_values is None:
-            prop_values = {}
-
+    def into_vars(self, prop_values: PropDict) -> BaseVars:
         def _create_var(key: str, var_info: _VarInfo) -> Var:
             ret = Var(var_info, None)
 
             if var_info.type is None or key not in prop_values:
                 return ret
 
-            prop = PropValue(var_info.type, prop_values.get(key))
+            if not isinstance(var_info.type, tOptional) and prop_values[key] is None:
+                return ret
+
+            prop = PropValue(var_info.type, prop_values[key])
             if prop.check():
                 ret._value = prop
             else:
@@ -219,3 +187,13 @@ def _create_var(key: str, var_info: _VarInfo) -> Var:
                 ]
 
         return BaseVars(ret_dict)
+
+
+@dataclass
+class BaseInputs(BaseVarInfos):
+    pass
+
+
+@dataclass
+class BaseOutputs(BaseVarInfos):
+    pass
diff --git a/src/spox/_function.py b/src/spox/_function.py
index d92a496..96a3127 100644
--- a/src/spox/_function.py
+++ b/src/spox/_function.py
@@ -86,10 +86,10 @@ def infer_output_types(self, input_prop_values: PropDict) -> dict[str, Type]:
 
         self.func_inputs = self.Inputs(**self.func_args)
         self.func_outputs = self.constructor(
-            self.func_attrs, self.func_inputs.vars(input_prop_values)
+            self.func_attrs, self.func_inputs.into_vars(input_prop_values)
         )
         self.func_graph = _graph.results(
-            **self.func_outputs._propagate_vars(input_prop_values).flatten_vars()
+            **self.func_outputs.into_vars(input_prop_values).flatten_vars()
         ).with_arguments(*func_args_var.values())
 
         return {
diff --git a/src/spox/_node.py b/src/spox/_node.py
index 35df2f6..8e2a022 100644
--- a/src/spox/_node.py
+++ b/src/spox/_node.py
@@ -267,7 +267,7 @@ def get_output_vars(
             self.validate_types()
 
         out_values = self.propagate_values(input_prop_values)
-        return self.outputs._propagate_vars(out_values)
+        return self.outputs.into_vars(out_values)
 
     def validate_types(self) -> None:
         """Validation of types, ran at the end of Node creation."""