Error in linkbutton event

I have a form with two panels, one panel contains fields for user registeration the other for sigining in, initially the registration panel is hidden, I have a linkbutton in the other panel to click on it in case of a new user, all I do in its click event is to hide the sign in panel & show the register panel

protected void registerBtn_Click(object sender, EventArgs e)
    {
        pnlSignIn.Visible = false;
        pnlRegister.Visible = true;
    }

the problem is when I click the button I receive the following error

Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page.  For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them.  If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

What's that supposed to mean?
I tried adding this  enableEventValidation="true"  to the page directive, but when I did that there was no error but the button now has to be clicked twice.

Any help please.