Why does my ajaxToolkit:MaskEditExtender cause a security exception on my local machine?

I'm using an ajaxToolkit:MaskEditExtender for my textbox, and it's throwing a security exception. i've included the markup. Any ideas?

<asp:TextBox runat="server" ID="txtStudentIdSearchBox" MaxLength="17"></asp:TextBox>
                <ajaxToolkit:MaskedEditExtender ID="meeStudentIDSearchBox" runat="server" TargetControlID="txtStudentIdSearchBox"
                    Mask="CCCCCCCCCCCCCCCCC" MessageValidatorTip="true" OnFocusCssClass="MaskedEditFocus"
                    OnInvalidCssClass="MaskedEditError" MaskType="None" InputDirection="LeftToRight"
                    AcceptNegative="None" DisplayMoney="None" ErrorTooltipEnabled="True" Filtered="*" />

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