diff --git a/Cortical_Column.h b/Cortical_Column.h
index 7c4ec60..01f96e7 100644
--- a/Cortical_Column.h
+++ b/Cortical_Column.h
@@ -191,7 +191,7 @@ class Cortical_Column {
                         x	= init(0.0);	/* derivative of y									*/
 
     /* Data storage access */
-    friend void get_data (int, Cortical_Column&, Thalamic_Column&, std::vector<double*>);
+    friend void get_data (int, Cortical_Column&, Thalamic_Column&, std::vector<double*>&);
 
     /* Stimulation protocol access */
     friend class Stim;