Insufficient number of outputs from right hand side of equal sign to satisfy assignment.

Hi , in order to detect cars in a video i'm using BoundingBox like this :

    frame1=im2bw(frame2,0.2); 
    [Labelimage]=bwlabeln(frame1); 
    stats=regionprops(Labelimage,'basic');
    BB=stats.BoundingBox;

But it keeps giving me the error stated in the title .

Thanks in advance !