diff --git a/lib/roken/parse_time.3 b/lib/roken/parse_time.3
index fbcce903b..d824e7691 100644
--- a/lib/roken/parse_time.3
+++ b/lib/roken/parse_time.3
@@ -107,7 +107,11 @@ and
 .Fn unparse_time_approx 
 return the number of characters written to 
 .Fa buf .
-.\".Sh EXAMPLES
+if the return value is greater than or equal to the
+.Fa len
+argument, the string was too short and some of the printed characters
+were discarded.
+.Sh EXAMPLES
 .Bd -literal
 #include <stdio.h>
 #include <parse_time.h>
diff --git a/lib/roken/parse_units.c b/lib/roken/parse_units.c
index f224814eb..d417fe23d 100644
--- a/lib/roken/parse_units.c
+++ b/lib/roken/parse_units.c
@@ -212,9 +212,13 @@ unparse_something (int num, const struct units *units, char *s, size_t len,
 	    tmp = (*print) (s, len, div, u->name, num);
 	    if (tmp < 0)
 		return tmp;
-
-	    len -= tmp;
-	    s += tmp;
+	    if (tmp > len) {
+		len = 0;
+		s = NULL;
+	    } else {
+		len -= tmp;
+		s += tmp;
+	    }
 	    ret += tmp;
 	}
     }