diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -341,8 +341,8 @@
     @length.validator
     def check_length(self, attribute, value):
         """Checks the length is positive."""
-        if value < 0:
-            raise ValueError('Length must be positive.')
+        if value < -1:
+            raise ValueError('Length must be positive or -1.')
 
     @reason.validator
     def check_reason(self, attribute, value):