diff --git a/mmcv/transforms/wrappers.py b/mmcv/transforms/wrappers.py
index fbc15c27b..5edc4a3c0 100644
--- a/mmcv/transforms/wrappers.py
+++ b/mmcv/transforms/wrappers.py
@@ -265,8 +265,9 @@ class KeyMapper(BaseTransform):
         return _map(data, remapping)
 
     def transform(self, results: Dict) -> Dict:
-
-        inputs = self.map_input(results, self.mapping)
+        inputs = results
+        if self.mapping:
+            inputs = self.map_input(inputs, self.mapping)
         outputs = self.transforms(inputs)
 
         if self.remapping:
@@ -368,8 +369,12 @@ class TransformBroadcaster(KeyMapper):
         # infer split number from input
         seq_len = None
         key_rep = None
-        for key in self.mapping:
+        if self.mapping:
+            keys = self.mapping.keys()
+        else:
+            keys = data.keys()
 
+        for key in keys:
             assert isinstance(data[key], Sequence)
             if seq_len is not None:
                 if len(data[key]) != seq_len:
@@ -383,14 +388,16 @@ class TransformBroadcaster(KeyMapper):
         scatters = []
         for i in range(seq_len):
             scatter = data.copy()
-            for key in self.mapping:
+            for key in keys:
                 scatter[key] = data[key][i]
             scatters.append(scatter)
         return scatters
 
     def transform(self, results: Dict):
         # Apply input remapping
-        inputs = self.map_input(results, self.mapping)
+        inputs = results
+        if self.mapping:
+            inputs = self.map_input(inputs, self.mapping)
 
         # Scatter sequential inputs into a list
         inputs = self.scatter_sequence(inputs)
diff --git a/tests/test_transforms/test_transforms_wrapper.py b/tests/test_transforms/test_transforms_wrapper.py
index d6b439d6b..1595edc2e 100644
--- a/tests/test_transforms/test_transforms_wrapper.py
+++ b/tests/test_transforms/test_transforms_wrapper.py
@@ -138,6 +138,15 @@ def test_cache_random_parameters():
 
 
 def test_key_mapper():
+    # Case 0: only remap
+    pipeline = KeyMapper(
+        transforms=[AddToValue(addend=1)], remapping={'value': 'v_out'})
+
+    results = dict(value=0)
+    results = pipeline(results)
+
+    np.testing.assert_equal(results['value'], 0)  # should be unchanged
+    np.testing.assert_equal(results['v_out'], 1)
 
     # Case 1: simple remap
     pipeline = KeyMapper(
@@ -313,6 +322,15 @@ def test_transform_broadcaster():
     np.testing.assert_equal(results['a'], 3)
     np.testing.assert_equal(results['b'], 7)
 
+    # Case 3: apply to all keys
+    pipeline = TransformBroadcaster(
+        transforms=[SumTwoValues()], mapping=None, remapping=None)
+    results = dict(num_1=[1, 2, 3], num_2=[4, 5, 6])
+
+    results = pipeline(results)
+
+    np.testing.assert_equal(results['sum'], [5, 7, 9])
+
     # Case 4: inconsistent sequence length
     with pytest.raises(ValueError):
         pipeline = TransformBroadcaster(