When exporting a table with a horizontal line the column count is wrong.

Output from pdflatex run:

! Extra alignment tab has been changed to \cr.
<template> \endtemplate 
                        
l.32 ...\multicolumn{4}{r}{Continued on next page}
                                                  \
? 

Here's the table in the tex file:

\begin{longtable}{||lll||}
\caption{This is a long table with lines around and between cells}\\
 Heading1  &  Heading2  &  Heading3 \\
\hline
\endhead
\hline\multicolumn{4}{r}{Continued on next page}\
\endfoot
\endlastfoot
\hline
 alpha     &  beta      &  gamma     \\
           &            &            \\
\end{longtable}

Here's the org file:

** table export test

#+CAPTION: This is a long table with lines around and between cells
#+LATEX_HEADER: \usepackage[landscape]{geometry}
#+LATEX_HEADER: \geometry{left=0.12in,right=0.12in,top=0.25in,bottom=0.25in}
#+ATTR_LaTeX: longtable align=||lll||

    | / | <30>     | <10>     | <10>     |
    |   | Heading1 | Heading2 | Heading3 |
    |---+----------+----------+----------|
    |   | alpha    | beta     | gamma    |
    |   |          |          |          |