diff --git a/microsim/interface.cpp b/microsim/interface.cpp
index da035e9..737da09 100755
--- a/microsim/interface.cpp
+++ b/microsim/interface.cpp
@@ -317,7 +317,7 @@ extern int open_cod_file(Processor **, char *);
 
 int Interface::open(const char *file)
 {
-    char *str;
+    const char *str;
 
     str = strrchr(file,'.');
     if(str==NULL)