I'm trying to calculate the twi from my area using LN((flowaccu+1)*900/Tan(sloperadian). My first result was wrong because my area is an endhoreic basin. So I made a flow accumulation raster based on a DEM where the collecting lake was cut out. This flow direction raster was true but the raster calculator refuses to do the last step. I refilled the lake in the flowaccu raster with the a slightly higer value than the original flowaccu raster (thus 40 000 000) so that there are no holes (NO more NoData values) and both the sloperadian raster and flowaccu raster are in the same projected coordinate system (WGS84 mercator auxilary sphere).I'm working in ArcMap 10.2 and also tried on 10.4 but the error message stays the same.
ERROR 000539: Error runing expression: rcexec() Traceback (post recent call last)