RequiredFieldValidator not preventing postback

I have this simple code:

        <asp:Label ID="litInfo" runat="server" meta:resourceKey="litInfo" />
        <br />
        <asp:TextBox ID="txtEmail" runat="server" /><br />
        <asp:RequiredFieldValidator ID="rfvEmail" runat="server" 
            ControlToValidate="txtEmail" Display="Dynamic" 
            ErrorMessage="zadejte e-mailovou adresu" Font-Bold="False" 
            SetFocusOnError="True" EnableClientScript="True" Font-Names="Tahoma" 
            Font-Size="Small" />
        <asp:RegularExpressionValidator ID="revEmail" runat="server" 
            ErrorMessage="spatny format e-mailove adresy" ControlToValidate="txtEmail" 
            Display="Dynamic" Font-Bold="False" 
            ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" 
            Font-Names="Tahoma" Font-Size="Small" />
        <br />
        <span style="margin-left: 230px;"><asp:Button ID="btnSubmit" runat="server" meta:resourceKey="btnSubmit" CausesValidation="True" /></span>
        <br />

I am expecting the RequiredFieldValidator control to prevent a postback when the textbox is filled in with an incorrect data and the button pressed. What happens is that the validator displays an error message but a postback is fired. Any suggestions why?