diff --git a/cause.h b/cause.h
index 809142e..a9c8a41 100644
--- a/cause.h
+++ b/cause.h
@@ -38,13 +38,13 @@
 #define CAUSE_UNIMPLEMENTED	79
 
 struct isdn_cause {
-	char *english;
-	char *german;
+	const char *english;
+	const char *german;
 };
 
 struct isdn_location {
-	char *english;
-	char *german;
+	const char *english;
+	const char *german;
 };
 
 extern struct isdn_cause isdn_cause[128];