diff --git a/src/cpp.c b/src/cpp.c
index d19d04c6b61c88160f7865db6389f9e7cda2b2d0..d7ecec95ec537976a6a8c53a796ea6648e274c8b 100644
--- a/src/cpp.c
+++ b/src/cpp.c
@@ -3593,7 +3593,7 @@ void init_cpp()
 }
 
 
-void add_predefine(char *s)
+void add_predefine(const char *s)
 {
   struct pike_predef_s *tmp=ALLOC_STRUCT(pike_predef_s);
   char * pos=STRCHR(s,'=');
diff --git a/src/cpp.h b/src/cpp.h
index 700cc813e17cf02263f01f6f0451df970805ebbc..ed5c8e8ae4d2261d6b1b2f7bd795c40861437b6f 100644
--- a/src/cpp.h
+++ b/src/cpp.h
@@ -15,7 +15,7 @@ struct hash_entry;
 /* Prototypes begin here */
 void f_cpp(INT32 args);
 void init_cpp(void);
-void add_predefine(char *s);
+void add_predefine(const char *s);
 void exit_cpp(void);
 /* Prototypes end here */
 #endif