Created Unassigned: set image.TransparencyColor2 cause error [26009]

Dear Sir,
I have updated mapwindow from v4.8 to v4.9, I got the following error when trying to run this line of code:

image.TransparencyColor2 = newColour;

//error message
Attempted to read or write protected memory. This is often an indication that other memory is corrupt

I am wondering if anyone has come across this error before?

thanks
Yong