From the documentation for Length constraint (http://symfony.com/doc/current/reference/constraints/Length.html)

Validates that a given string length is between some minimum and maximum value.

For min option:

Validation will fail if the given value's length is less than this min value.

Nothing special is said about empty strings. It seems reasonable to assume that if you validate an empty string against Length constraint with min = 1 it should fail.

But in reality, emty string passes validation.