VSTO C# Outlook 2007.

Does anyone know why I am getting the following exception from the code
below and what the solution is?

"Exception while setting up user property:
A custom field with this name but a different data type already exists.
Enter a different name.
Prop name = x-MYAPP-MyDate1
Value=21/11/2009 3:01:30 a.m.
Type = olDateTime"


private void SetProperty(string propertyName, object value,
Outlook.OlUserPropertyType propType)
{
try
{
//Create user property if does not exist
if (mail.UserProperties[propertyName] == null)
{
mail.UserProperties.Add(propertyName, propType,
true, Type.Missing);
}

//Set property value
if (value != null)
{
mail.UserProperties[propertyName].Value = value;
}
//Delete the property if value was null
else
{
mail.UserProperties[propertyName].Delete();
}
}
catch (Exception e)
{
string msg = "Exception while setting up user property:\n" +
e.Message + "\n"
+ "Prop name = " + propertyName + "\nValue=" +
value.ToString() + "\nType = " + propType;

MYAPP.LogMessage(msg);

}
}