Set Null value for raster datasets

Hi,

I am writing a Python code to automate a process. In it I need to use SetNull function several times to extracting the required values from the raster dataset.
I am using the following line of code, and the tool is running without any error; but it is not providing the desired output.

arcpy.gp.RasterCalculator_sa("SetNull(ZMAXRas == -99,ZMAXRas)",outWorkspace + inputASCII [:-4] + "_W")

Where, ZMAXRas is the input raster variable; outWorkspace and inputASCII are the others variables, those I have used for to retain the naming convention of my output data.

Can anyone please suggest why I am not getting the raster without '-99' value after running the script.

For your information, I am using ArcGIS 10.1.

Thanks in advance.
Trina.