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
--- /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
--- a/src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php
+++ b/src/markup/engine/remarkup/blockrule/PhutilRemarkupSimpleTableBlockRule.php
@@ -73,7 +73,14 @@
         // 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';
           }
         }
       }