Nee help with error: "not all code paths return a value"

Hi,Why does this function bring the error "not all code paths return a value"?
How do I cover every code path?  public bool ValidateNewUser(string strEmail) {

SqlConnection con = new SqlConnection(conStr);
SqlCommand cmd = new SqlCommand("sp_checkEmail", con);
cmd.CommandType = CommandType.StoredProcedure;

SqlParameter objParam1 = default(SqlParameter);
SqlParameter objReturnParam = default(SqlParameter);

objParam1 = cmd.Parameters.Add("@Email", SqlDbType.VarChar);
objReturnParam = cmd.Parameters.Add("@Duplicates", SqlDbType.Int);
objReturnParam ...