Using contains field lookup with Substr database function cause modification of second parameter of Substr

[norac89]

My model:

class User(models.Model):
    field1 = models.Charfield(max_length=120)
    field2 = models.Charfield(max_length=120)

Tried querying the model to get User instances in which field2 start with the three first characters of field1.
Tried the following:

User.objects.filter(field2__startswith=Substr(F('field1'), 1, 3))

I m getting the following error

psycopg2.DataError: invalid input syntax for integer: "1%"
LINE 1: ...(REPLACE(REPLACE((SUBSTRING("test_user"."field1", '1%', 3)),...

The second parameter of the substr function seems to be replaced with '1%'.