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I'm trying to use some of the GRASS tools from the processing toolbox in QGIS. I get the following Python error:

An error has occured while executing Python code:

Traceback (most recent call last):
  File "C:/PROGRA~1/QGISCH~1/apps/qgis/./python/plugins\processing\gui\ProcessingToolbox.py", line 181, in executeAlgorithm
    message = alg.checkBeforeOpeningParametersDialog()
  File "C:/PROGRA~1/QGISCH~1/apps/qgis/./python/plugins\processing\algs\grass7\Grass7Algorithm.py", line 511, in checkBeforeOpeningParametersDialog
    msg = Grass7Utils.checkGrass7IsInstalled()
  File "C:/PROGRA~1/QGISCH~1/apps/qgis/./python/plugins\processing\algs\grass7\Grass7Utils.py", line 361, in checkGrass7IsInstalled
    cmdpath = os.path.join(path, 'bin', 'r.out.gdal.exe')
  File "C:\PROGRA~1\QGISCH~1\apps\Python27\lib\ntpath.py", line 96, in join
    assert len(path) > 0
TypeError: object of type 'NoneType' has no len()

Python version: 2.7.4

QGIS version: 2.4.0-Chugiak

Any ideas on how I can fix this? I've tried a re-installing but haven't had any luck - same error keeps on reappearing.