Dynamics & simultaneous notes yield "programming error"

[Joel C. Salomon]

Compiling the attached file yields errors of the sort

> programming error: asked to compute volume at +1.000000 for dynamic span of 
> duration 1.000000 starting at 1
> continuing, cross fingers

If I replace the construct

    << { \voiceOne
        a1 |
    } \new Voice { \voiceTwo
        a,1 |
    } >> \oneVoice |

with

    << {a1} \\ {a,1} >>

this error goes away.

Can someone please explain what I’m doing wrong here?

—Joel C. Salomon

programming_error.ly
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