diff --git a/yt/units/tests/test_ytarray.py b/yt/units/tests/test_ytarray.py
index 1081bd7b747..121f71fde1e 100644
--- a/yt/units/tests/test_ytarray.py
+++ b/yt/units/tests/test_ytarray.py
@@ -815,7 +815,8 @@ def unary_ufunc_comparison(ufunc, a):
     elif ufunc is np.invert:
         assert_raises(TypeError, ufunc, a)
     elif hasattr(np, 'isnat') and ufunc is np.isnat:
-        assert_raises(ValueError, ufunc, a)
+        # numpy 1.13 raises ValueError, numpy 1.14 and newer raise TypeError
+        assert_raises((TypeError, ValueError), ufunc, a)
     else:
         # There shouldn't be any untested ufuncs.
         assert_true(False)
diff --git a/yt/units/yt_array.py b/yt/units/yt_array.py
index 76ebfe30e56..7ec60c797cf 100644
--- a/yt/units/yt_array.py
+++ b/yt/units/yt_array.py
@@ -529,7 +529,7 @@ def __str__(self):
         """
 
         """
-        return super(YTArray, self).__str__()+' '+self.units.__str__()
+        return str(self.view(np.ndarray)) + ' ' + str(self.units)
 
     #
     # Start unit conversion methods