diff --git a/modules/control_system/functions/@tf/minus.m b/modules/control_system/functions/@tf/minus.m
index 0783489e7c..e290358e63 100644
--- a/modules/control_system/functions/@tf/minus.m
+++ b/modules/control_system/functions/@tf/minus.m
@@ -8,9 +8,9 @@
 % LICENCE_BLOCK_END
 %=============================================================================
 function sys = minus(sysA, sysB)
-  sys_tfA = tf(sysA);
-  sys_tfB = tf(sysB);
-  if ~issiso(sysA) || ~issiso(sysA)
+  sysA = tf(sysA);
+  sysB = tf(sysB);
+  if ~issiso(sysA) || ~issiso(sysB)
     error(_('SISO lti expected.'));
   end
   
diff --git a/modules/control_system/functions/@tf/plus.m b/modules/control_system/functions/@tf/plus.m
index c2a8017c6b..bbf2fef060 100644
--- a/modules/control_system/functions/@tf/plus.m
+++ b/modules/control_system/functions/@tf/plus.m
@@ -8,9 +8,9 @@
 % LICENCE_BLOCK_END
 %=============================================================================
 function sys = plus(sysA, sysB)
-  sys_tfA = tf(sysA);
-  sys_tfB = tf(sysB);
-  if ~issiso(sysA) || ~issiso(sysA)
+  sysA = tf(sysA);
+  sysB = tf(sysB);
+  if ~issiso(sysA) || ~issiso(sysB)
     error(_('SISO lti expected.'));
   end
   
@@ -39,7 +39,6 @@
   if ~isempty(UserData)
     sys.UserData = UserData;
   end
-  
 end
 %=============================================================================
 function sys = plus_siso(sysA, sysB)