KSP 1.8.1: PartCompiler error on "category = None"

[Gordon Dry]

Parts with "category = None" are not just hidden inside the VAB, they even throw an exception like this:

PartLoader: Encountered exception during compilation. System.ArgumentException: Requested value 'None' was not found.
  at System.Enum+EnumResult.SetFailure (System.Enum+ParseFailureKind failure, System.String failureMessageID, System.Object failureMessageFormatArgument) [0x00023] in <ad04dee02e7e4a85a1299c7ee81c79f6>:0 
  at System.Enum.TryParseEnum (System.Type enumType, System.String value, System.Boolean ignoreCase, System.Enum+EnumResult& parseResult) [0x0017a] in <ad04dee02e7e4a85a1299c7ee81c79f6>:0 
  at System.Enum.Parse (System.Type enumType, System.String value, System.Boolean ignoreCase) [0x00010] in <ad04dee02e7e4a85a1299c7ee81c79f6>:0 
  at System.Enum.Parse (System.Type enumType, System.String value) [0x00000] in <ad04dee02e7e4a85a1299c7ee81c79f6>:0 
  at ConfigNode.ParseEnum (System.Type enumType, System.String vectorString) [0x00000] in <9d71e4043e394d78a6cf9193ad011698>:0 
  at ConfigNode.ReadValue (System.Type fieldType, System.String value) [0x00516] in <9d71e4043e394d78a6cf9193ad011698>:0 
  at ConfigNode.ReadObject (System.Object obj, ConfigNode node) [0x00143] in <9d71e4043e394d78a6cf9193ad011698>:0 
  at ConfigNode.LoadObjectFromConfig (System.Object obj, ConfigNode node, System.Int32 pass, System.Boolean removeAfterUse) [0x0001a] in <9d71e4043e394d78a6cf9193ad011698>:0 
  at PartLoader.ParsePart (UrlDir+UrlConfig urlConfig, ConfigNode node) [0x001a9] in <9d71e4043e394d78a6cf9193ad011698>:0 
  at PartLoader+<CompileParts>d__56.MoveNext () [0x005cd] in <9d71e4043e394d78a6cf9193ad011698>:0 
 
(Filename: C:\buildslave\unity\build\Runtime/Export/Debug/Debug.bindings.h Line: 35)

PartCompiler: Cannot compile part