Error: (assq) bad argument type: #f

        Call history:

        mowedline.scm:334: xtypes#xrectangle-x    
        mowedline.scm:336: xtypes#xrectangle-width        
        mowedline.scm:337: coops#slot-value       
        mowedline.scm:338: xlib#xrectinregion     
        mowedline.scm:339: L      
        mowedline.scm:349: last   
        mowedline.scm:350: coops#slot-value       
        mowedline.scm:351: xtypes#xrectangle-x    
        mowedline.scm:352: xtypes#xrectangle-width        
        mowedline.scm:353: xtypes#xrectangle-x    
        mowedline.scm:354: xtypes#xrectangle-width        
        mowedline.scm:359: xlib#xflush    
        mowedline.scm:789: thread-start!          
        mowedline.scm:790: thread-start!          
        mowedline.scm:791: dbus-eventloop         
        mowedline.scm:779: dbus#poll-for-message                <--