diff --git a/bidict/_common.py b/bidict/_common.py
index 1104b61..0fe6f4f 100644
--- a/bidict/_common.py
+++ b/bidict/_common.py
@@ -56,10 +56,14 @@ class BidirectionalMapping(Mapping):
         Returns True if only its start is not None and False if only its stop
         is not None.
         """
-        if slice.step is not None or \
-            (not ((slice.start is None) ^ (slice.stop is None))):
+        start_missing = slice.start is None
+        start_found = not start_missing
+        stop_missing = slice.stop is None
+        step_found = slice.step is not None
+
+        if step_found or start_missing == stop_missing:
             raise TypeError('Slice must specify only either start or stop')
-        return slice.start is not None
+        return start_found
 
     def __getitem__(self, keyorslice):
         """
diff --git a/tests/test_slice.py b/tests/test_slice.py
new file mode 100644
index 0000000..bc2f4cb
--- /dev/null
+++ b/tests/test_slice.py
@@ -0,0 +1,49 @@
+from itertools import product
+
+import pytest
+
+from bidict import bidict
+
+
+@pytest.fixture
+def b():
+    return bidict(H='hydrogen')
+
+single_test_data = {'H', 'hydrogen', -1, 0, 1}
+bad_start_values = single_test_data - {'H'}
+bad_stop_values = single_test_data - {'hydrogen'}
+pair_test_data = product(single_test_data, single_test_data)
+
+
+def test_good_start(b):
+    assert b['H'] == 'hydrogen'
+    assert b['H':] == 'hydrogen'
+
+@pytest.mark.parametrize('start', bad_start_values)
+def test_bad_start(b, start):
+    with pytest.raises(KeyError):
+        b[start]
+    with pytest.raises(KeyError):
+        b[start:]
+
+def test_good_stop(b):
+    assert b[:'hydrogen'] == 'H'
+
+@pytest.mark.parametrize('stop', bad_stop_values)
+def test_stop(b, stop):
+    with pytest.raises(KeyError):
+        b[:stop]
+
+@pytest.mark.parametrize('start, stop', pair_test_data)
+def test_start_stop(b, start, stop):
+    with pytest.raises(TypeError):
+        b[start:stop]
+
+@pytest.mark.parametrize('step', single_test_data)
+def test_step(b, step):
+    with pytest.raises(TypeError):
+        b[::step]
+
+def test_empty(b):
+    with pytest.raises(TypeError):
+        b[::]