diff --git a/spb/graphics/control.py b/spb/graphics/control.py
index b746a0e..2515f74 100644
--- a/spb/graphics/control.py
+++ b/spb/graphics/control.py
@@ -530,10 +530,13 @@ def _set_lower_upper_limits(system, lower_limit, upper_limit,
         lower_limit = 0
     if not upper_limit:
         tfinal = None
-        if not kwargs.get("params", None):
+        ct = import_module("control")
+        if ct and (not kwargs.get("params", None)):
+            # this procedure requires the control module
             control_sys = tf_to_control(system)
             tfinal, _ = _ideal_tfinal_and_dt(control_sys, is_step=is_step)
-        # if interactive widget plot, use default value of 10
+        # if interactive widget plot, or control is not installed, use
+        # default value of 10
         upper_limit = tfinal if tfinal else 10
     return lower_limit, upper_limit
 
diff --git a/spb/utils.py b/spb/utils.py
index 2d9bca6..dbe3776 100644
--- a/spb/utils.py
+++ b/spb/utils.py
@@ -756,7 +756,7 @@ def _from_sympy_to_ct(num, den):
             )
         return ct.tf(n, d)
 
-    if isinstance(tf, ct.TransferFunction):
+    if ct and isinstance(tf, ct.TransferFunction):
         return tf
 
     if isinstance(tf, Expr):
@@ -780,7 +780,7 @@ def _from_sympy_to_ct(num, den):
             den.append(row_den)
         return ct.tf(num, den)
 
-    elif isinstance(tf, sp.signal.TransferFunction):
+    elif sp and isinstance(tf, sp.signal.TransferFunction):
         return ct.tf(tf.num, tf.den, dt=0 if tf.dt is None else tf.dt)
 
     elif isinstance(tf, (list, tuple)):