diff --git a/src/libiso8601/400_manip.h b/src/libiso8601/400_manip.h
index 2f45e2e..57f3a39 100644
--- a/src/libiso8601/400_manip.h
+++ b/src/libiso8601/400_manip.h
@@ -148,7 +148,7 @@ stored in \a sign (if it is not a null pointer), i.e. -1 if \a d2 > \a d1 or
 void iso8601_difference(const struct iso8601_date* d1,
     const struct iso8601_date* d2, struct iso8601_elapsed* per, int* sign)
 #ifndef DOXYGEN
-    __attribute__((nonnull))
+    __attribute__((nonnull(1,2)))
 #endif
 ;