Imager 1.6.4 and Craft 2.7.7 - An unknown image object was used

[marc524]

Something changed between Craft 2.7.6 and 2.7.7 (changelog just says: "Updated Twig to 1.38.4") that broke Imager reading string URLs. My template is now returning An unknown image object was used

Not sure where to post version 1 support questions for Craft 2. Apologies if this is not the correct spot.

My assets are on S3 and so my code calls an image with:
{% set heroImage = entry.heroImage.first.url() %}

before going into the whole
{% set transformedImages = craft.imager.transformImage(heroImage, ...) %}

And now, for whatever reason, this isn't getting read as a string anymore on line 26 of Imager_ImagePathsModel.php

I can role back to Craft 2.7.6 and everything works fine.
Using Imager 1.6.4