qwtPlot crashes if data of spectromplot is constant

If I apply absolut constant data to the RasterData of a spectrogramplot 
the application crashes if I try to view the plot.
All my code is passed, but only if I bring the plot to the foreground 
the application crashes.
I cannot debugthe problem, since the whole code used at that point is 
within the qwt dll.

The error is:
ASSERT failure in QVector<T>::operator[]:"index out of range", file 
c:\.../qvector.h, line 320

Matthias

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