I’m guessing you have a NUL byte in input.csv. You can test that with
if '\0' in open('input.csv').read():
print "you have null bytes in your input file"
else:
print "you don't"
if you do,
reader = csv.reader(x.replace('\0', '') for x in mycsv)
may get you around that. Or it may indicate you have utf16 or something ‘interesting’ in the .csv file.