diff --git a/src/charles.cc b/src/charles.cc
index 63e1b36..975ef9f 100644
--- a/src/charles.cc
+++ b/src/charles.cc
@@ -9,7 +9,7 @@
 
 #include "basics.h"
 
-char *OUT_FILE = "charles_out.png";
+const char *OUT_FILE = "charles_out.png";
 
 
 int