Ensure direction hint has valid value

theano/sandbox/cuda/dnn.py

@@ -975,6 +975,10 @@ def dnn_conv(img, kerns, border_mode='valid', subsample=(1, 1),
       capability of 3.0 or higer.  This means that older GPU will not
       work with this Op.
     """
+
+    # Ensure the value of direction_hint is supported
+    assert direction_hint in [None, 'bprop weights', 'forward']
+
     fgraph = getattr(img, 'fgraph', None) or getattr(kerns, 'fgraph', None)
     if (border_mode == 'valid' and subsample == (1, 1) and
         direction_hint == 'bprop weights'):
@@ -1059,6 +1063,10 @@ def dnn_conv3d(img, kerns, border_mode='valid', subsample=(1, 1, 1),
     :warning: dnn_conv3d only works with cuDNN library 3.0
 
     """
+
+    # Ensure the value of direction_hint is supported
+    assert direction_hint in [None, 'bprop weights', 'forward']
+
     fgraph = getattr(img, 'fgraph', None) or getattr(kerns, 'fgraph', None)
     if (border_mode == 'valid' and subsample == (1, 1, 1) and
         direction_hint == 'bprop weights'):