diff -u b/includes/errors.inc b/includes/errors.inc
--- b/includes/errors.inc
+++ b/includes/errors.inc
@@ -210,7 +210,7 @@
     $number++;
   }
 
-  $st_opts = variable_get('error_backtrace_display', array());
+  $st_opts = variable_get('error_backtrace_display', array(2 => 0, 1 => 0));
   if (array_sum($st_opts)) {
     $error['!backtrace'] = format_backtrace($backtrace);
   }
@@ -360,7 +360,7 @@
  *   An HTML string.
  */
 function format_backtrace(array $backtrace) {
-  $st_opts = variable_get('error_backtrace_display', array());
+  $st_opts = variable_get('error_backtrace_display', array(2 => 0, 1 => 0));
 
   $show_vars = (variable_get('error_backtrace_display_scalar', ERROR_REPORTING_DISPLAY_SCALAR) ||
                 variable_get('error_backtrace_display_non_scalar', ERROR_REPORTING_DISPLAY_NON_SCALAR));
diff -u b/modules/system/system.admin.inc b/modules/system/system.admin.inc
--- b/modules/system/system.admin.inc
+++ b/modules/system/system.admin.inc
@@ -1684,7 +1684,7 @@
   $form['backtrace_options']['error_backtrace_display'] = array(
     '#type' => 'checkboxes',
     '#title' => t('Choose how to monitor backtrace information.'),
-    '#default_value' => variable_get('error_backtrace_display', array()),
+    '#default_value' => variable_get('error_backtrace_display', array(2 => 0, 1 => 0)),
     '#options' => array(
       ERROR_REPORTING_DISPLAY_MESSAGES => t('Show on page'),
       ERROR_REPORTING_DISPLAY_LOGS => t('Add to log'),