diff --git a/lib/graphlib.php b/lib/graphlib.php
index 6c79f3478a4..4e193f37a83 100644
--- a/lib/graphlib.php
+++ b/lib/graphlib.php
@@ -978,7 +978,7 @@ function init_y_axis() {
   $axis_size       = $this->parameter['axis_size'];
   $axis_colour     = $this->parameter['axis_colour'];
   $axis_angle      = $this->parameter['y_axis_angle'];
-  $y_tick_labels   = $this->y_tick_labels;
+  $y_tick_labels   = $this->parameter['y_tick_labels'];
 
   $this->calculated['y_axis_left']['has_data'] = FALSE;
   $this->calculated['y_axis_right']['has_data'] = FALSE;