diff --git a/src/markup/engine/__tests__/remarkup/table-with-long-header.txt b/src/markup/engine/__tests__/remarkup/table-with-long-header.txt
new file mode 100644
index 0000000..84b5cb8
--- /dev/null
+++ b/src/markup/engine/__tests__/remarkup/table-with-long-header.txt
@@ -0,0 +1,8 @@
+|x|
+||--
+~~~~~~~~~~
+<div class="remarkup-table-wrap"><table class="remarkup-table">
+<tr><td>x</td></tr>
+</table></div>
+~~~~~~~~~~
+| x |
diff --git a/src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php b/src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php
index 58ad3d5..72aae08 100644
--- a/src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php
+++ b/src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php
@@ -1,89 +1,96 @@
 <?php
 
 final class PhutilRemarkupSimpleTableBlockRule extends PhutilRemarkupBlockRule {
 
   public function getMatchingLineCount(array $lines, $cursor) {
     $num_lines = 0;
     while (isset($lines[$cursor])) {
       if (preg_match('/^(\s*\|.*+\n?)+$/', $lines[$cursor])) {
         $num_lines++;
         $cursor++;
       } else {
         break;
       }
     }
 
     return $num_lines;
   }
 
   public function markupText($text, $children) {
     $matches = array();
 
     $rows = array();
     foreach (explode("\n", $text) as $line) {
       // Ignore ending delimiters.
       $line = rtrim($line, '|');
 
       // NOTE: The complexity in this regular expression allows us to match
       // a table like "| a | [[ href | b ]] | c |".
 
       preg_match_all(
         '/\|'.
         '('.
           '(?:'.
             '(?:\\[\\[.*?\\]\\])'. // [[ ... | ... ]], a link
             '|'.
               '(?:[^|[]+)'.          // Anything but "|" or "[".
             '|'.
               '(?:\\[[^\\|[])'.      // "[" followed by anything but "[" or "|"
           ')*'.
         ')/', $line, $matches);
 
       $any_header = false;
       $any_content = false;
 
       $cells = array();
       foreach ($matches[1] as $cell) {
         $cell = trim($cell);
 
         // If this row only has empty cells and "--" cells, and it has at
         // least one "--" cell, it's marking the rows above as <th> cells
         // instead of <td> cells.
 
         // If it has other types of cells, it's always a content row.
 
         // If it has only empty cells, it's an empty row.
 
         if (strlen($cell)) {
           if (preg_match('/^--+\z/', $cell)) {
             $any_header = true;
           } else {
             $any_content = true;
           }
         }
 
         $cells[] = array('type' => 'td', 'content' => $this->applyRules($cell));
       }
 
       $is_header = ($any_header && !$any_content);
 
       if (!$is_header) {
         $rows[] = array('type' => 'tr', 'content' => $cells);
       } else if ($rows) {
         // Mark previous row with headings.
         foreach ($cells as $i => $cell) {
           if ($cell['content']) {
-            $rows[last_key($rows)]['content'][$i]['type'] = 'th';
+            $last_key = last_key($rows);
+            if (!isset($rows[$last_key]['content'][$i])) {
+              // If this row has more cells than the previous row, there may
+              // not be a cell above this one to turn into a <th />.
+              continue;
+            }
+
+            $rows[$last_key]['content'][$i]['type'] = 'th';
           }
         }
       }
     }
 
     if (!$rows) {
       return $this->applyRules($text);
     }
 
     return $this->renderRemarkupTable($rows);
   }
 
 }