diff --git a/Source/Modules/guile.cxx b/Source/Modules/guile.cxx
index c7e8fe7e4..0777b316f 100644
--- a/Source/Modules/guile.cxx
+++ b/Source/Modules/guile.cxx
@@ -280,7 +280,7 @@ public:
     // Make sure `prefix' ends in an underscore
     if (prefix) {
       const char *px = Char(prefix);
-      if (px[Len(prefix)] != '_')
+      if (px[Len(prefix) - 1] != '_')
 	Printf(prefix, "_");
     }
 
diff --git a/Source/Modules/mzscheme.cxx b/Source/Modules/mzscheme.cxx
index c2abd7ec4..e2f44deef 100644
--- a/Source/Modules/mzscheme.cxx
+++ b/Source/Modules/mzscheme.cxx
@@ -101,10 +101,9 @@ public:
     }
 
     // If a prefix has been specified make sure it ends in a '_' (not actually used!)
-
     if (prefix) {
       const char *px = Char(prefix);
-      if (px[Len(prefix)] != '_')
+      if (px[Len(prefix) - 1] != '_')
 	Printf(prefix, "_");
     } else
       prefix = NewString("swig_");
diff --git a/Source/Modules/ocaml.cxx b/Source/Modules/ocaml.cxx
index 9fc5e9c71..beac03237 100644
--- a/Source/Modules/ocaml.cxx
+++ b/Source/Modules/ocaml.cxx
@@ -130,10 +130,9 @@ public:
     }
 
     // If a prefix has been specified make sure it ends in a '_' (not actually used!)
-
     if (prefix) {
       const char *px = Char(prefix);
-      if (px[Len(prefix)] != '_')
+      if (px[Len(prefix) - 1] != '_')
 	Printf(prefix, "_");
     } else
       prefix = NewString("swig_");