\hhline 断开顶部的垂直线

Question

我创建了一个表格，其中想要一条双水平线，所以我使用了 \hhline

\begin{table}[ht]
\begin{tabular}{|l|l|}
\hline
\textbf{Message} & \multicolumn{1}{c|}{\textbf{Description}}                      \\ \hline
ACK             & Confirms an INVITE request                                      \\ \hline
BYE             & Ends a session                                                  \\ \hline
CANCEL          & Cancels establishing of a session                               \\ \hline
INVITE          & Establishes a session                                           \\ \hline
OPTIONS         & Cpabilities of the calling and receiving SIP phones             \\ \hline
REGISTER        & Registers the user and updates the location table               \\ \hhline{|=|=|}
NOTIFY          & Sends information after subscribing                             \\ \hline
PUBLISH         & Uploads the status information to the server                    \\ \hline
SUBSCRIBE       & Establishes a session to receive future updates                 \\ \hline
\end{tabular}
\caption{\label{tab:tablename}SIP methods.}
\end{table}

是什么原因导致我的桌子顶部的垂直线与最顶部的水平线断开：

有什么办法可以解决这个问题吗？提前感谢您的帮助。