diff --git a/lib/subordinateio.c b/lib/subordinateio.c
index 7fddf62f..597aeac5 100644
--- a/lib/subordinateio.c
+++ b/lib/subordinateio.c
@@ -623,17 +623,28 @@ bool have_sub_uids(const char *owner, uid_t start, unsigned long count)
 	return have_range (&subordinate_uid_db, owner, start, count);
 }
 
+/*
+ * sub_uid_add: add a subuid range, perhaps through nss.
+ *
+ * Return 1 if the range is already present or on success.  On error
+ * return 0 and set errno appropriately.
+ */
 int sub_uid_add (const char *owner, uid_t start, unsigned long count)
 {
-	if (get_subid_nss_handle())
-		return -EOPNOTSUPP;
+	if (get_subid_nss_handle()) {
+		errno = EOPNOTSUPP;
+		return 0;
+	}
 	return add_range (&subordinate_uid_db, owner, start, count);
 }
 
+/* Return 1 on success.  on failure, return 0 and set errno appropriately */
 int sub_uid_remove (const char *owner, uid_t start, unsigned long count)
 {
-	if (get_subid_nss_handle())
-		return -EOPNOTSUPP;
+	if (get_subid_nss_handle()) {
+		errno = EOPNOTSUPP;
+		return 0;
+	}
 	return remove_range (&subordinate_uid_db, owner, start, count);
 }
 
@@ -719,17 +730,28 @@ bool local_sub_gid_assigned(const char *owner)
 	return range_exists (&subordinate_gid_db, owner);
 }
 
+/*
+ * sub_gid_add: add a subgid range, perhaps through nss.
+ *
+ * Return 1 if the range is already present or on success.  On error
+ * return 0 and set errno appropriately.
+ */
 int sub_gid_add (const char *owner, gid_t start, unsigned long count)
 {
-	if (get_subid_nss_handle())
-		return -EOPNOTSUPP;
+	if (get_subid_nss_handle()) {
+		errno = EOPNOTSUPP;
+		return 0;
+	}
 	return add_range (&subordinate_gid_db, owner, start, count);
 }
 
+/* Return 1 on success.  on failure, return 0 and set errno appropriately */
 int sub_gid_remove (const char *owner, gid_t start, unsigned long count)
 {
-	if (get_subid_nss_handle())
-		return -EOPNOTSUPP;
+	if (get_subid_nss_handle()) {
+		errno = EOPNOTSUPP;
+		return 0;
+	}
 	return remove_range (&subordinate_gid_db, owner, start, count);
 }