diff --git a/Assets/VRM10/Runtime/IO/Material/BuiltInRP/Import/Materials/BuiltInVrm10MToonMaterialImporter.cs b/Assets/VRM10/Runtime/IO/Material/BuiltInRP/Import/Materials/BuiltInVrm10MToonMaterialImporter.cs
index 7e682c69d..2f07ad27b 100644
--- a/Assets/VRM10/Runtime/IO/Material/BuiltInRP/Import/Materials/BuiltInVrm10MToonMaterialImporter.cs
+++ b/Assets/VRM10/Runtime/IO/Material/BuiltInRP/Import/Materials/BuiltInVrm10MToonMaterialImporter.cs
@@ -244,6 +244,7 @@ namespace UniVRM10
         {
             switch (material?.alphaMode)
             {
+                case null:
                 case "OPAQUE":
                     return MToon10AlphaMode.Opaque;
                 case "MASK":