Images before sequence default to first sequence frame ?

I have a big problem with inserting masking image sequences into my animation. The situation is as follows: I have a sequence of 50 images used as masks that I want to use for masking from frame 435 to 485, so I insert the sequence and setup the masking nodes. What happens? All frames before frame 435 (start of the masking) are filled up with the image which is the first in the masking range. Shouldn’t the mask just be non-existent before frame 435, or at least be set to a black transparant image.
The way sequences are used now are impossible to use with masking of a range of frames. Only option I see is padding the start of the frame with empty images, what a drag… Certainly when you are some thousand frames into an animation…

Solution found: convert all jpeg masks to png (this format can contain transparancy information), then add one extra frame to the beginning of the sequence containing a transparant frame. This frame will now be used as default before the sequence upto frame one of the sequence.