Incompatibilities of cmap with fontenc/hyperref?

The following code doesn't compile (with pdflatex)

\documentclass{article}\usepackage{cmap}\usepackage[T1]{fontenc}\usepackage[pdfa=true]{hyperref}\begin{document}Hello.\end{document}

unless I omit either the fontenc-line or the pdfa=true-option from the hyperref-line. The error message I get says: ! pdfTeX error (setup): \pdfminorversion cannot be changed after data is written to the PDF file. Is there something I can do or is this an incompatibility?