Cannot merge sets

I'm getting the following exception while contructing my display with
several fields with different time domains:

visad.VisADException: Cannot merge sets
    at visad.Set.merge1DSets(
    at visad.DataShadow.setAnimationSampling(
    at visad.SimpleSet.setAnimationSampling(
    at visad.SampledSet.computeRanges(
    at visad.FieldImpl.computeRanges(
    at visad.DataRenderer.computeRanges(
    at visad.DataRenderer.prepareAction(
    at visad.DisplayRenderer.prepareAction(
    at visad.DisplayImpl.doAction(
    at visad.util.ThreadPool$
visad.VisADError: visad.VisADException: Cannot merge sets
    at visad.util.ThreadPool$

It is not consistant (and neither is my data) so I'm wondering if it is
due to the nature of my data. I've looked at the relevant bit of Set

    // find indices of set not covered by this
    int set_length = set.getLength();
    boolean[] set_indices = new boolean[set_length];
    for (int i=0; i<set_length; i++) set_indices[i] = true;
    if (set_length > 1) {
      // set indices for values in this
      int[] test_indices = set.doubleToIndex(values);
      try {
        for (int i=0; i<length; i++) {
          if (test_indices[i] > -1) set_indices[test_indices[i]]
      } catch (ArrayIndexOutOfBoundsException aioobe) {
        throw new VisADException("Cannot merge sets");

but don't quite understand what conditions would lead to this exception.

Let me know if there is something obvious, otherwise I'll try to make a
reproducible case for debugging.


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