diff --git a/skimage/morphology/greyreconstruct.py b/skimage/morphology/greyreconstruct.py
index bce26d6b..ec7aa182 100644
--- a/skimage/morphology/greyreconstruct.py
+++ b/skimage/morphology/greyreconstruct.py
@@ -152,6 +152,9 @@ def reconstruction(seed, mask, method='dilation', selem=None, offset=None):
         pad_value = np.min(seed)
     elif method == 'erosion':
         pad_value = np.max(seed)
+    else:
+        raise ValueError("Reconstruction method can be one of 'erosion' "
+                         "or 'dilation'. Got '%s'." % method)
     images = np.ones(dims) * pad_value
     images[[0] + inside_slices] = seed
     images[[1] + inside_slices] = mask