diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index c5d8e85..adae96b 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -473,7 +473,7 @@
 
   /* Called to emit a diagnostic if client_diagnostic option is true.
      This callback receives the translated message.  */
-  void (*error) (cpp_reader *, int, const char *, va_list)
+  void (*error) (cpp_reader *, int, const char *, va_list *)
        ATTRIBUTE_PRINTF(3,0);
 };