Index: tol/btol/bmodel/model.cpp
===================================================================
--- tol/btol/bmodel/model.cpp	(revision fa3cdedc5c68e9ce609325e3e4b8d089d0fc7d12)
+++ tol/btol/bmodel/model.cpp	(revision 9180d5263e00f27b3295d756465eb1612310b030)
@@ -283,4 +283,11 @@
       }
 	    transfer_[n] = Pol(inp[1]);
+      if( transfer_[n][0].Degree()<0)
+      {
+    		Error(I2("Transfer function cannot have negative degree terms (F)",
+		          	 "la función de transferencia no puede tener términos de grado negativo (F)"));
+		    aborted_ = BTRUE;
+		    return;
+      }
 	    inpName_ [n] = inp[2]->Identify();
 	    inpName_ [n].Replace('\n',' ');