Index: mttroot/mtt/lib/control/PPP/ppp_ystar.m
==================================================================
--- mttroot/mtt/lib/control/PPP/ppp_ystar.m
+++ mttroot/mtt/lib/control/PPP/ppp_ystar.m
@@ -43,10 +43,15 @@
       error("U must be a column vector with n_u*n_U components");
     endif
   else
     n_u = length(U)/n_U;	# Deduce n_u from U if no system
   endif
+  
+  [n_x0,m_x0] = size(x_0);
+  if n_x0<>n_x
+    error(sprintf("x_0 must be a column with length %i", n_x));
+  endif
   
 
   [n,m]=size(tau);
   if (n != 1 )
     error("tau must be a row vector of times");