Problem: Spatial Analyst Histogram tool does not work when a selection is present on a floating-point raster
Creating a histogram using the Histogram tool on the Spatial Analyst toolbar causes a fatal error if there is a table attribute selection present on a floating-point raster.
This is a known issue with ArcGIS 8.1.
Solution or Workaround
Convert the floating-point raster to integer by typing the following line in the Raster Calculator:
Create a histogram based on a selection of the values of the integer raster.
Alternatively, install Service Pack 2.