setting associatedcontrolid tag fails

i have multiple control adds textbox tag control the controls collection. i try set label's associatedcontrolid clientid textbox i error

unable control id 
'ctl00_maincontentplaceholder_matrixsetcontrol_mec50_tb'
that compared tag 'lb'. 

ok small background. i got main-composite control boldly adds array 'elements' the control collection. elements occur 'matrixtextbox' control consisting textbox label.

i reason tag textbox stable category variables init createchildcontrols:

    elementtextbox = new textbox();
    elementtextbox.id = "tb";
    controls.add(elementtextbox);

    elementlabel = new label();
    elementlabel.id = "lb";
    controls.add(elementlabel);

i attempted sourroundings the

elementlabel.associatedcontrolid = elementtextbox.clientid;

both right after adding controls controls collection even prerender - both furnish same error. am i doing wrong?