HDR Merge - Image Metadata missing

Hi,

I have some images (PNGs) that want to create a HDR Merge from, however when selecting them and starting HDR merge, get error message...."Unable to merge the photos. The files lack the neccessary Metadata for merging".

Any idea what part of the metadata this is trying to find?

Is there a way of adding the data or pre-processing them somehow in LR or PS to recreate the data?

I tried exporting them to jpg first, but get same error?

HDR Merge works ok with other images of mine in past.

Cheers,

Nick