Index: wp-includes/admin-bar/admin-bar-debug.php
===================================================================
--- wp-includes/admin-bar/admin-bar-debug.php	(revision 15986)
+++ wp-includes/admin-bar/admin-bar-debug.php	(working copy)
@@ -139,7 +139,7 @@
 
 	$query_count = '<h2><span>Total Queries:</span>' . number_format( $wpdb->num_queries ) . "</h2>\n";
 	$query_time = '<h2><span>Total query time:</span>' . number_format(sprintf('%0.1f', $total_time * 1000), 1) . "ms</h2>\n";
-	$memory_usage = '<h2><span>Peak Memory Used:</span>' . number_format( memory_get_peak_usage( ) ) . " bytes</h2>\n";
+	$memory_usage = '<h2><span>Peak Memory Used:</span>' . (function_exists('memory_get_peak_usage') ? number_format( memory_get_peak_usage( ) ) : 'Unavailable') . " bytes</h2>\n";
 
 	$out = $query_count . $query_time . $memory_usage . $out;