Hi,

I have the following minimal not-working example:

\starttext
\startitemize
\item One
  \startmode[modeA]
  \item Two
    \startitemize
      \startmode[modeB] % <- this
      \item Two A       % <- does
      \stopmode         % <- not work
      % \doifmode{modeB}{\item Two A} % <- this works
    \item Two B
    \stopitemize
  \stopmode
\stopitemize  
\stoptext

Note that both modeA and modeB are not defined and hence not enabled.
When compiled with the latest version of ConTeXt MkIV, I get: Missing number, treated as zero.
However, If I use the inline version \doifmode instead of the environment version \startmode \stopmode, then the example compiles without problem.

I am afraid but this may be a bug.

Cheers,
Christoph