Importing products with dashes in SKUs

This is a strange problem - I've imported a few thousand products correctly (more or less), and now I am trying to update their categories through the product import function using their SKU and new Category name.

However, I get an error saying "Record X: does not contain a product SKU" for any record whose SKU contains a hyphen, such as PS-SSD3.
However, I don't know why the hyphen would be a problem now, since those same products were imported fine the first time. I also don't know how to get around it if it is a problem.

Can anyone help please?? This is one of the last steps to getting my client's site launched!
Thank you!