diff --git a/source/include/signalflow/node/operators/select-input.h b/source/include/signalflow/node/operators/select-input.h
index 76437a4c..20895152 100644
--- a/source/include/signalflow/node/operators/select-input.h
+++ b/source/include/signalflow/node/operators/select-input.h
@@ -9,7 +9,9 @@ namespace signalflow
 {
 
 /**--------------------------------------------------------------------------------*
- * Generates the
+ * Pass through the output of one or more `inputs`, based on the integer input index
+ * specified in `index`. Unlike `ChannelSelect`, inputs may be multichannel,
+ * and `index` can be modulated in real time.
  *---------------------------------------------------------------------------------*/
 class SelectInput : public VariableInputNode
 {
diff --git a/tests/test_node_operators.py b/tests/test_node_operators.py
index c993e5b0..fbd2c01b 100644
--- a/tests/test_node_operators.py
+++ b/tests/test_node_operators.py
@@ -225,3 +225,22 @@ def test_select_input(graph):
     assert np.all(a.output_buffer[0][4:8] == 2)
     assert np.all(a.output_buffer[0][8:12] == 3)
     assert np.all(a.output_buffer[0][12:16] == 1)
+
+    # test multichannel upmix behaviour -
+    # should always honour the max number of input channels
+    a = ChannelArray([1, 2])
+    b = Constant(3)
+    c = SelectInput([a, b], index=0)
+    assert a.num_output_channels == 2
+    assert b.num_output_channels == 1
+    assert c.index.num_output_channels == 1
+    assert c.num_output_channels == 2
+
+    graph.render_subgraph(c, reset=True)
+    assert np.all(c.output_buffer[0] == 1)
+    assert np.all(c.output_buffer[1] == 2)
+
+    c.index = 1
+    graph.render_subgraph(c, reset=True)
+    assert np.all(c.output_buffer[0] == 3)
+    assert np.all(c.output_buffer[1] == 3)