diff --git a/control/iosys.py b/control/iosys.py
index df75f3b54..49ab29c77 100644
--- a/control/iosys.py
+++ b/control/iosys.py
@@ -366,8 +366,8 @@ def _rhs(self, t, x, u):
         you may want to use :meth:`dynamics`.
 
         """
-        NotImplemented("Evaluation not implemented for system of type ",
-                       type(self))
+        raise NotImplementedError("Evaluation not implemented for system of type ",
+                                  type(self))
 
     def dynamics(self, t, x, u, params=None):
         """Compute the dynamics of a differential or difference equation.
diff --git a/control/modelsimp.py b/control/modelsimp.py
index 432b76b96..b1c1ae31c 100644
--- a/control/modelsimp.py
+++ b/control/modelsimp.py
@@ -476,7 +476,7 @@ def markov(Y, U, m=None, transpose=False):
 
     # Make sure there is enough data to compute parameters
     if m > n:
-        warn.warning("Not enough data for requested number of parameters")
+        warnings.warn("Not enough data for requested number of parameters")
 
     #
     # Original algorithm (with mapping to standard order)
diff --git a/control/optimal.py b/control/optimal.py
index 4913cc341..de45a8dd0 100644
--- a/control/optimal.py
+++ b/control/optimal.py
@@ -20,8 +20,6 @@
 from .exception import ControlNotImplemented
 from .timeresp import TimeResponseData
 
-__all__ = ['find_optimal_input']
-
 # Define module default parameter values
 _optimal_defaults = {
     'optimal.minimize_method': None,
diff --git a/control/statesp.py b/control/statesp.py
index a2fa3dd73..af549cff6 100644
--- a/control/statesp.py
+++ b/control/statesp.py
@@ -59,6 +59,8 @@
 from scipy.signal import cont2discrete
 from scipy.signal import StateSpace as signalStateSpace
 from warnings import warn
+
+from .exception import ControlSlycot
 from .frdata import FrequencyResponseData
 from .lti import LTI, _process_frequency_response
 from .namedio import common_timebase, isdtime
diff --git a/control/tests/optimal_test.py b/control/tests/optimal_test.py
index b100e7e14..cdea19bfa 100644
--- a/control/tests/optimal_test.py
+++ b/control/tests/optimal_test.py
@@ -381,17 +381,8 @@ def test_optimal_logging(capsys):
 @pytest.mark.parametrize("fun, args, exception, match", [
     [opt.quadratic_cost, (np.zeros((2, 3)), np.eye(2)), ValueError,
      "Q matrix is the wrong shape"],
-    [opt.quadratic_cost, (np.eye(2), 1), ValueError,
+    [opt.quadratic_cost, (np.eye(2), np.eye(2, 3)), ValueError,
      "R matrix is the wrong shape"],
-])
-def test_constraint_constructor_errors(fun, args, exception, match):
-    """Test various error conditions for constraint constructors"""
-    sys = ct.ss2io(ct.rss(2, 2, 2))
-    with pytest.raises(exception, match=match):
-        fun(sys, *args)
-
-
-@pytest.mark.parametrize("fun, args, exception, match", [
     [opt.input_poly_constraint, (np.zeros((2, 3)), [0, 0]), ValueError,
      "polytope matrix must match number of inputs"],
     [opt.output_poly_constraint, (np.zeros((2, 3)), [0, 0]), ValueError,