diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index 76c297a..6c4225c 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -248,7 +248,7 @@
 };
 
 /* Say which field is in use.  */
-extern enum cpp_token_fld_kind cpp_token_val_index (cpp_token *tok);
+extern enum cpp_token_fld_kind cpp_token_val_index (const cpp_token *tok);
 
 /* A type wide enough to hold any multibyte source character.
    cpplib's character constant interpreter requires an unsigned type.