From 320d01a325670e88da0d16aad8f6843577425629 Mon Sep 17 00:00:00 2001
From: marko1olo <barsukdana@gmail.com>
Date: Sat, 6 Jun 2026 08:33:07 +0400
Subject: [PATCH] Fix zero-input interconnect D matrix shape

Preserve explicitly shaped empty state-space matrices so linearized interconnections with zero inputs and nonzero outputs keep a (noutputs, 0) D matrix.

Co-authored-by: OpenAI Codex <codex@openai.com>
---
 control/statesp.py                 |  8 ++++----
 control/tests/interconnect_test.py | 30 ++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/control/statesp.py b/control/statesp.py
index 65529b99d..16981e62b 100644
--- a/control/statesp.py
+++ b/control/statesp.py
@@ -2295,7 +2295,7 @@ def _ssmatrix(data, axis=1, square=None, rows=None, cols=None, name=None):
 
     Returns
     -------
-    arr : 2D array, with shape (0, 0) if a is empty
+    arr : 2D array, with shape (0, 0) for empty vectors or matrices
 
     """
     # Process the name of the object, if available
@@ -2310,9 +2310,9 @@ def _ssmatrix(data, axis=1, square=None, rows=None, cols=None, name=None):
     if (ndim > 2):
         raise ValueError(f"state-space matrix{name} must be 2-dimensional")
 
-    elif (ndim == 2 and shape == (1, 0)) or \
-         (ndim == 1 and shape == (0, )):
-        # Passed an empty matrix or empty vector; change shape to (0, 0)
+    elif (ndim == 2 and shape == (0, 0)) or \
+            (ndim == 1 and shape == (0, )):
+        # Passed a 0-by-0 matrix or empty vector; change shape to (0, 0)
         shape = (0, 0)
 
     elif ndim == 1:
diff --git a/control/tests/interconnect_test.py b/control/tests/interconnect_test.py
index ccce76f34..e725b29f6 100644
--- a/control/tests/interconnect_test.py
+++ b/control/tests/interconnect_test.py
@@ -432,6 +432,36 @@ def test_linear_interconnect():
         outlist=['plant.y'], outputs='y')
     assert clsys.syslist[0].name == 'ctrl'
 
+
+def test_interconnect_zero_input_one_output():
+    plant = ct.ss(
+        [[1, 1], [0, -2]],
+        [[0], [1]],
+        [[1, 0]],
+        [[0]],
+        inputs=['u'],
+        outputs=['y'],
+        name='plant',
+        dt=True,
+    )
+    controller = ct.ss(
+        [[2.25, 1], [-5, -0.5]],
+        [[-1.25], [4.5]],
+        [[-0.5, 1.5]],
+        [[0]],
+        inputs=['y'],
+        outputs=['u'],
+        name='controller',
+        dt=True,
+    )
+
+    sys = ct.interconnect([plant, controller], inplist=None, outlist=['y'])
+
+    assert sys.ninputs == 0
+    assert sys.noutputs == 1
+    assert sys.D.shape == (1, 0)
+
+
 @pytest.mark.parametrize(
     "connections, inplist, outlist, inputs, outputs", [
         pytest.param(