Cannot save all of the property settings for this Web Part – Sharepoint

"Cannot save all of the property settings for this Web Part. The
default namespace "http://schemas.microsoft.com/WebPart/v2" is a
reserved namespace for base Web Part properties. Custom Web Part
properties require a unique namespace (specified through an
XmlElementAttribute on the property, or an XmlRootAttribute on the
class)."

No where do I get help regarding this error.

This is when adding custom properties to my webpart, why cant I save the properties when I edit my webpart and click on save/apply? (then I get that error)

Code–

     [DefaultProperty("Text"), ToolboxData("<{0}:CustomPropertyWebPart runat=server></{0}:CustomPropertyWebPart>"),
            XmlRoot(Namespace = "ExecuteStoreProc")]
            public class CustomPropertyWebPart : Microsoft.SharePoint.WebPartPages.WebPart
            {
                const string c_MyStringDefault = "Sample String";
            }

            // Create a custom category in the property sheet.
            [Category("Custom Properties")]
            // Assign the default value.
            [DefaultValue(c_MyStringDefault)]
            // Property is available in both Personalization
            // and Customization mode.
            [WebPartStorage(Storage.Personal)]
            // The caption that appears in the property sheet.
            [FriendlyNameAttribute("Custom String")]
            // The tool tip that appears when pausing the mouse pointer over
        // the friendly name in the property pane.
        [Description("Type a string value.")]
        // Display the property in the property pane.
        [Browsable(true)]
        [XmlElement(ElementName = "MyString")]

        // The accessor for this property.
        public string MyString
        {
            get
            {
                return _myString;
            }
            set
            {
                _myString = value;
            }
        }