I know it's nearly Christmas, but at my desk it's Spring(4D) again. I am getting the exception

I know it's nearly Christmas, but at my desk it's Spring(4D) again. I am getting the exception

"Type is missing a guid: Systematic.BaseTypes.DevExp.TFieldFactory."

So what I have (simplified) is below and the call stack at the end

I've checked that all interfaces have GUIDs. Any suggestions greatly appreciated

Interface

Type

IMyField = interface
['{7A54F949-C1C9-474C-B848-3234FBAAB48E}']
{procs and stuff}
end;

TMyFieldBase = class(TInterfacedObject, IMyField)
public
constructor Create(aOwner: TComponent; aGroupName: string);
end;

TMyFieldOne = class(TMyFieldBase)
end;

TMyFieldTwo = class(TMyFieldBase)
end;

Implementation
var MyContainer: TContainer;

Type
{M+}
TFieldFactory = reference to function(AOwner: TComponent;
aGroupName: string): IDevExCustomField;
{$M-}

Initialization

MyContainer := TContainer.Create;

MyContainer.RegisterType('One');
MyContainer.RegisterType('Two');
MyContainer.RegisterFactory.AsFactory('One');
MyContainer.RegisterFactory.AsFactory('Two');
MyContainer.Build;
end.

Here's the MadExcept call stack

main thread ($252c):
00930edb +73 SMXPHM.exe Spring.Container.Registration TComponentRegistry.CheckIsNonGuidInterface
00930f38 +34 SMXPHM.exe Spring.Container.Registration TComponentRegistry.Validate
0093112e +5a SMXPHM.exe Spring.Container.Registration TComponentRegistry.RegisterService
00935972 +32 SMXPHM.exe Spring.Container.Registration TRegistration.Implements
035a36a0 +34 SMXPHM.exe Systematic.BaseTypes.DevExp 842 +0 {Spring.Container.Registration}TRegistration.Implements
035a475b +33 SMXPHM.exe Systematic.BaseTypes.DevExp 2147 +0 {Spring.Container.Registration}TRegistration.Implements
035a4546 +4e SMXPHM.exe Systematic.BaseTypes.DevExp 1575 +0 TContainer.RegisterType
035a4046 +2e SMXPHM.exe Systematic.BaseTypes.DevExp 1719 +0 TContainer.RegisterFactory
0364753c +c8 SMXPHM.exe Systematic.BaseTypes.DevExp 468 +12 initialization