Document has an e-Signature, but after save-as the signature is invalid.

The PDF comes up in Acrobat XI showing OK, but when checking the signature the signature suddenly becomes "invalid" even though I've done nothing to modify it. This only happens if I do a Save As to the document first.

Sample code:

fieldname = jso.getnthfieldname(0)

Set signatureOne = jso.getField(fieldname)

ostate = signatureOne.signatureValidate()   'signature is magically broken here--says document is corrupt or modified