diff --git a/lib/roken/parse_units.h b/lib/roken/parse_units.h
index 7c4462fb2..a2df70db8 100644
--- a/lib/roken/parse_units.h
+++ b/lib/roken/parse_units.h
@@ -65,6 +65,10 @@ parse_flags (const char *s, const struct units *units,
 size_t
 unparse_units (int num, const struct units *units, char *s, size_t len);
 
+size_t
+unparse_units_approx (int num, const struct units *units, char *s,
+		      size_t len);
+
 size_t
 unparse_flags (int num, const struct units *units, char *s, size_t len);