ExecuteError in append command

I'm trying to append several shapefiles together. I adapted an example script from http://help.arcgis.com/en/arcgisdesk...l#//0013000000, but it's not working. Moreover, the error code is completely uninformative to me. Here's the script:

# Import system modules
import arcpy
from arcpy import env

# Set environment settings
env.workspace = "C:\Users\deryugin\Downloads\backup"

# Set local variables
inCovers = ["tr01_d00.shp", "tr02_d00.shp", "tr04_d00.shp", "tr05_d00.shp"]
outCover = "test.shp"
appendMethod = ""
featureClasses = ""
numberMethod = ""

# Execute Append
arcpy.Append_arc(inCovers, outCover, appendMethod, featureClasses, numberMethod)

Here is the error output:

Traceback (most recent call last):
File "C:\Users\deryugin\Dropbox\LR effects of pollution\analysis\data\Census tract shapefiles\append_files.py", line 22, in <module>
arcpy.Append_arc(inCovers, outCover, appendMethod, featureClasses, numberMethod)
File "C:\Program Files (x86)\ArcGIS\Desktop10.0\arcpy\arcpy\arc.py", line 1399, in Append
raise e
ExecuteError: ERROR 999999: Error executing function.
Failed to execute (Append).

Any ideas about what I'm doing wrong?