diff --git a/control/iosys.py b/control/iosys.py
index 895e04015..16ef633b7 100644
--- a/control/iosys.py
+++ b/control/iosys.py
@@ -855,7 +855,7 @@ def __call__(sys, u, params=None, squeeze=None):
 
         # Evaluate the function on the argument
         out = sys._out(0, np.array((0,)), np.asarray(u))
-        _, out = _process_time_response(sys, [], out, [], squeeze=squeeze)
+        _, out = _process_time_response(sys, None, out, None, squeeze=squeeze)
         return out
 
     def _update_params(self, params, warning=False):
@@ -1867,8 +1867,10 @@ def linearize(sys, xeq, ueq=[], t=0, params={}, **kw):
 
 # Utility function to parse a signal parameter
 def _parse_signal_parameter(value, name, kwargs, end=False):
+    # Check kwargs for a variant of the parameter name
     if value is None and name in kwargs:
-        value = list(kwargs.pop(name))
+        value = kwargs.pop(name)
+
     if end and kwargs:
         raise TypeError("unknown parameters %s" % kwargs)
     return value
diff --git a/control/timeresp.py b/control/timeresp.py
index 55ced8302..9ccf24bf3 100644
--- a/control/timeresp.py
+++ b/control/timeresp.py
@@ -467,8 +467,11 @@ def _process_time_response(
 
     Parameters
     ----------
+    sys : LTI or InputOutputSystem
+        System that generated the data (used to check if SISO/MIMO).
+
     T : 1D array
-        Time values of the output
+        Time values of the output.  Ignored if None.
 
     yout : ndarray
         Response of the system.  This can either be a 1D array indexed by time
@@ -478,9 +481,9 @@ def _process_time_response(
 
     xout : array, optional
         Individual response of each x variable (if return_x is True). For a
-        SISO system (or if a single input is specified), This should be a 2D
+        SISO system (or if a single input is specified), this should be a 2D
         array indexed by the state index and time (for single input systems)
-        or a 3D array indexed by state, input, and time.
+        or a 3D array indexed by state, input, and time. Ignored if None.
 
     transpose : bool, optional
         If True, transpose all input and output arrays (for backward
@@ -545,7 +548,7 @@ def _process_time_response(
         raise ValueError("unknown squeeze value")
 
     # Figure out whether and how to squeeze the state data
-    if issiso and len(xout.shape) > 2:
+    if issiso and xout is not None and len(xout.shape) > 2:
         xout = xout[:, 0, :]            # remove input
 
     # See if we need to transpose the data back into MATLAB form
@@ -555,7 +558,8 @@ def _process_time_response(
 
         # For signals, put the last index (time) into the first slot
         yout = np.transpose(yout, np.roll(range(yout.ndim), 1))
-        xout = np.transpose(xout, np.roll(range(xout.ndim), 1))
+        if xout is not None:
+            xout = np.transpose(xout, np.roll(range(xout.ndim), 1))
 
     # Return time, output, and (optionally) state
     return (tout, yout, xout) if return_x else (tout, yout)