Callback evaluation error using Simulink Masked Subsystem Dialog Callbacks

I am using dialog callbacks to validate data entry, As an example I have created a subsystem containing a Gain block with parameter G and entered the following in subsystem mask, parameters, dialog callback;

if str2num(get_param(gcb, 'G'))<0 error('Gain is negative.') end

now when I enter a negative value for the Gain I get the following;

--> Error evaluating 'MaskCallback' callback of Subsystem block (mask) '<a href="matlab:open_and_hilite_system ('untitled/Subsystem')">untitled/Subsystem</a>'. --> Gain is negative

Can anyone tell me why I get the error evaluating the callback?

I am running R2011a on Mac OS X Mountain Lion

Thanks

[Kaustubha Govind]

AFK, that is the format in which errors thrown from the the mask callback. Perhaps you could submit an enhancement request if you'd like them to be reported differently?

[Chris Emerson]

OK thanks, I was only expecting to see '--> Gain is negative' on the error dialog, maybe this has already been 'enhanced' with the additional text?!

[Kaustubha Govind]

Chris: Yes, I think your error message is appended to a standard message.