3.1 Runtime Error

Strange - I CLEAN and compile one of my data fields. It builds fine, but fails at run time in the simulator with this:

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Error: System Error
Details: Failed invoking <symbol>
Stack:
  - onBackgroundData() at C:\SHARED_VMWARE\ConnectIQ\MyProjects\CyclingDashboard\source\CyclingDashboard.mc:130 0x1000054b

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The offending line of code is:

        WatchUi.requestUpdate();

as the last line in this function:

   function onBackgroundData(data) {

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However, without making ANY code changes, I simply RUN it again, and it works fine.

I can recreate this sequence by CLEANING the project. CLEAN, RUN (fails), RUN (works)

Any ideas?