1 files changed, 27 insertions, 2 deletions
diff --git a/Lib/fontTools/designspaceLib/types.py b/Lib/fontTools/designspaceLib/types.py
index 8afea96c..80ba9d6d 100644
--- a/Lib/fontTools/designspaceLib/types.py
+++ b/Lib/fontTools/designspaceLib/types.py
@@ -1,12 +1,15 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from typing import Dict, List, Optional, Union
+from typing import Dict, List, Optional, Union, cast
 
 from fontTools.designspaceLib import (
+    AxisDescriptor,
     DesignSpaceDocument,
+    DesignSpaceDocumentError,
     RangeAxisSubsetDescriptor,
     SimpleLocationDict,
+    ValueAxisSubsetDescriptor,
     VariableFontDescriptor,
 )
 
@@ -89,6 +92,10 @@ def userRegionToDesignRegion(doc: DesignSpaceDocument, userRegion: Region) -> Re
     designRegion = {}
     for name, value in userRegion.items():
         axis = doc.getAxis(name)
+        if axis is None:
+            raise DesignSpaceDocumentError(
+                f"Cannot find axis named '{name}' for region."
+            )
         if isinstance(value, (float, int)):
             designRegion[name] = axis.map_forward(value)
         else:
@@ -107,16 +114,34 @@ def getVFUserRegion(doc: DesignSpaceDocument, vf: VariableFontDescriptor) -> Reg
     #  - it's a single location = use it to know which rules should apply in the VF
     for axisSubset in vf.axisSubsets:
         axis = doc.getAxis(axisSubset.name)
-        if isinstance(axisSubset, RangeAxisSubsetDescriptor):
+        if axis is None:
+            raise DesignSpaceDocumentError(
+                f"Cannot find axis named '{axisSubset.name}' for variable font '{vf.name}'."
+            )
+        if hasattr(axisSubset, "userMinimum"):
+            # Mypy doesn't support narrowing union types via hasattr()
+            # TODO(Python 3.10): use TypeGuard
+            # https://mypy.readthedocs.io/en/stable/type_narrowing.html
+            axisSubset = cast(RangeAxisSubsetDescriptor, axisSubset)
+            if not hasattr(axis, "minimum"):
+                raise DesignSpaceDocumentError(
+                    f"Cannot select a range over '{axis.name}' for variable font '{vf.name}' "
+                    "because it's a discrete axis, use only 'userValue' instead."
+                )
+            axis = cast(AxisDescriptor, axis)
             vfUserRegion[axis.name] = Range(
                 max(axisSubset.userMinimum, axis.minimum),
                 min(axisSubset.userMaximum, axis.maximum),
                 axisSubset.userDefault or axis.default,
             )
         else:
+            axisSubset = cast(ValueAxisSubsetDescriptor, axisSubset)
             vfUserRegion[axis.name] = axisSubset.userValue
     # Any axis not mentioned explicitly has a single location = default value
     for axis in doc.axes:
         if axis.name not in vfUserRegion:
+            assert isinstance(
+                axis.default, (int, float)
+            ), f"Axis '{axis.name}' has no valid default value."
             vfUserRegion[axis.name] = axis.default
     return vfUserRegion