I am trying to extract a raster through a particular shapefile.


       import arcpy
       from arcpy import env
       from arcpy.sa import *
       arcpy.CheckOutExtension("Spatial")
       env.overwriteOutput = True

       #Set the current workspace
       env.workspace = ("C:\thesis\Forests Only Percent Treecover\2003 BSWM Forest")
       env.nodata = "MINIMUM"
       env.compression = "LZ77"

       #This is for extracting the forest areas of hansen percent treecover
       for raster in arcpy.ListRasters("*0E.tif", "TIFF"):
           print raster #checking the presence of raster
           outputRasterExtractbyMask = ExtractByMask(raster, "NAMRIA.shp")
           outputRasterExtractbyMask_Name = "forests_only"+raster
           outputRasterExtractbyMask.save(outputRasterExtractbyMask_Name)

       print "Finish"


As I print the raster in arcpy.ListRasters to check the presence of raster, this error shows up:
      
     Traceback (most recent call last):
     File "C:\Users\brentiebark\Dropbox\Python Scripts and mxds\extractbymask.py", line 14, in <module>
     for raster in arcpy.ListRasters("*0E.tif", "TIFF"):
     TypeError: 'NoneType' object is not iterable

I double checked the directory, and my files are really there. Is there something wrong with my code?