When in 'Train a Deep Learning Model', the process stops midway through with the Runtime Error: "Expected a non-empty list of Tensors". What does this mean and how do I fix this?

[ElianeMcCarthy]

I'm trying to train a deep learning model from thermal drone imagery, following along with the 'Use Deep Learning to assess palm tree health' tutorial. At the Training step, I get this error part way through the epochs: "Runtimeerror: expected a non-empty list of tensors" (see snip below). Does any one know how to troubleshoot this error?