diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index d84cdbf..2784c28 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1547,7 +1547,7 @@ function theme_table($variables) {
 
       // Flag the cell as a header and remove the flag.
       $is_header = TRUE;
-      $is_header |= isset($cell['header']);
+      $is_header = isset($cell['header']) ? $cell['header'] : TRUE;
       unset($cell['header']);
 
       // Track responsive classes for each column as needed. Only the header