autobeaming problem

[John Sankey]

I've got a piece that switches between 2/2 and 3/4 time signatures, so tried
\translator{\VoiceContext beamAutoBegin = 0;beamAutoEnd = "1/4";}
in the \paper block and
\property Voice.beamAutoEnd_8 = "1/2";
alternating with
\property Voice.beamAutoEnd_8 = "3/4";
in the Voice (thread) parts.

Lily objects to the _8 :
K282.ly:17:28: error: parse error, expecting `'='':
\property Voice.beamAutoEnd_
                            8 = "1/2";

How should I do it?
John