Gluoncv HybridBlock SRGAN demo cannot be hybridized?

I’m trying to put my hands on a trained SRGAN (full artifact, not just coeffs), and using the official gluoncv SRGAN script:

I’d like to serialize the generator (netG), and despite the generator being a HybridBlock, netG.hybridize() returns this error: AttributeError: 'Symbol' object has no attribute 'shape'…

What is wrong? how to hybridize the model? how can I persist the full model post training?