/series/import/request/{id}: inappropriate "Value must not be empty" error

[php-coder]

When I fill "http://a" as the value of the "Image URL" field, I got 2 errors one of them confusing:

[php-coder]

"Value must not be empty" error comes from @NotNull annotation on the downloadedImage field.
"Could not download file" error comes from interceptor that binds error message to the downloadedImage field.

I couldn't quickly find out a solution for it.