fers490

**Describe the bug** I'm getting the following error when trying to sign a document: ``` File ".venv/lib/python3.11/site-packages/pyhanko/sign/signers/functions.py", line 150, in async_sign_pdf return await pdf_signer.async_sign_pdf( ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File ".venv/lib/python3.11/site-packages/pyhanko/sign/signers/pdf_signer.py", line 1505, in...