diff --git a/ChangeLog b/ChangeLog
index 92dc644fa..18d103340 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2005-02-02  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
+	* src/tgbaalgos/randomgraph.cc (random_graph): Make sure n > 0.
+	* src/tgbatest/randtgba.cc: Check the range of all arguments.
+
 	* bench/emptchk/models/eeaean1.pml: New file, from schwoon.05.tacas.
 	* bench/emptchk/Makefile.am: Distribute models/eeaean1.pml
 	and build models/eeaean1.tgba.
diff --git a/src/tgbaalgos/randomgraph.cc b/src/tgbaalgos/randomgraph.cc
index bf4904e8c..3575f5d7a 100644
--- a/src/tgbaalgos/randomgraph.cc
+++ b/src/tgbaalgos/randomgraph.cc
@@ -1,4 +1,4 @@
-// Copyright (C) 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2004, 2005  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -94,6 +94,7 @@ namespace spot
 	       int n_acc, float a, float t,
 	       ltl::environment* env)
   {
+    assert(n > 0);
     tgba_explicit* res = new tgba_explicit(dict);
 
     int props_n = ap->size();
diff --git a/src/tgbaalgos/randomgraph.hh b/src/tgbaalgos/randomgraph.hh
index fec64cb3f..7f776a61c 100644
--- a/src/tgbaalgos/randomgraph.hh
+++ b/src/tgbaalgos/randomgraph.hh
@@ -1,4 +1,4 @@
-// Copyright (C) 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
+// Copyright (C) 2004, 2005  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
 // et Marie Curie.
 //
@@ -33,7 +33,7 @@ namespace spot
   /// \brief Construct a tgba randomly.
   /// \ingroup tgba_misc
   ///
-  /// \param n The number of states wanted in the automata.  All states
+  /// \param n The number of states wanted in the automata (>0).  All states
   ///          will be connected, and there will be no dead state.
   /// \param d The density of the automata.  This is the probability
   ///          (between 0.0 and 1.0), to add a transition between two
diff --git a/src/tgbatest/randtgba.cc b/src/tgbatest/randtgba.cc
index 99bc2e96e..4d1118158 100644
--- a/src/tgbatest/randtgba.cc
+++ b/src/tgbatest/randtgba.cc
@@ -214,6 +214,7 @@ syntax(char* prog)
   exit(2);
 }
 
+
 int
 to_int(const char* s)
 {
@@ -227,6 +228,32 @@ to_int(const char* s)
   return res;
 }
 
+int
+to_int_pos(const char* s, const char* arg)
+{
+  int res = to_int(s);
+  if (res <= 0)
+    {
+      std::cerr << "argument of " << arg
+		<< " (" << res << ") must be positive" << std::endl;
+      exit(1);
+    }
+  return res;
+}
+
+int
+to_int_nonneg(const char* s, const char* arg)
+{
+  int res = to_int(s);
+  if (res < 0)
+    {
+      std::cerr << "argument of " << arg
+		<< " (" << res << ") must be nonnegative" << std::endl;
+      exit(1);
+    }
+  return res;
+}
+
 float
 to_float(const char* s)
 {
@@ -241,6 +268,19 @@ to_float(const char* s)
   return res;
 }
 
+float
+to_float_nonneg(const char* s, const char* arg)
+{
+  float res = to_float(s);
+  if (res < 0)
+    {
+      std::cerr << "argument of " << arg
+		<< " (" << res << ") must be nonnegative" << std::endl;
+      exit(1);
+    }
+  return res;
+}
+
 struct ec_stat
 {
   int max_tgba_states;
@@ -1086,14 +1126,14 @@ main(int argc, char** argv)
 	{
 	  if (argc < argn + 3)
 	    syntax(argv[0]);
-	  opt_n_acc = to_int(argv[++argn]);
-	  opt_a = to_float(argv[++argn]);
+	  opt_n_acc = to_int_nonneg(argv[++argn], "-a");
+	  opt_a = to_float_nonneg(argv[++argn], "-a");
 	}
       else if (!strcmp(argv[argn], "-d"))
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_d = to_float(argv[++argn]);
+	  opt_d = to_float_nonneg(argv[++argn], "-d");
 	}
       else if (!strcmp(argv[argn], "-D"))
 	{
@@ -1105,7 +1145,7 @@ main(int argc, char** argv)
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_ec = to_int(argv[++argn]);
+	  opt_ec = to_int_nonneg(argv[++argn], "-e");
 	}
       else if (!strcmp(argv[argn], "-g"))
 	{
@@ -1140,7 +1180,7 @@ main(int argc, char** argv)
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_n = to_int(argv[++argn]);
+	  opt_n = to_int_pos(argv[++argn], "-n");
 	}
       else if (!strcmp(argv[argn], "-O"))
 	{
@@ -1171,20 +1211,20 @@ main(int argc, char** argv)
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_R = to_int(argv[++argn]);
+	  opt_R = to_int_pos(argv[++argn], "-R");
 	}
       else if (!strcmp(argv[argn], "-s"))
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_ec_seed = to_int(argv[++argn]);
+	  opt_ec_seed = to_int_nonneg(argv[++argn], "-s");
 	  spot::srand(opt_ec_seed);
 	}
       else if (!strcmp(argv[argn], "-t"))
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_t = to_float(argv[++argn]);
+	  opt_t = to_float_nonneg(argv[++argn], "-t");
 	}
       else if (!strcmp(argv[argn], "-z"))
 	{
@@ -1202,13 +1242,13 @@ main(int argc, char** argv)
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_f = to_int(argv[++argn]);
+	  opt_f = to_int_pos(argv[++argn], "-f");
 	}
       else if (!strcmp(argv[argn], "-F"))
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_F = to_int(argv[++argn]);
+	  opt_F = to_int_nonneg(argv[++argn], "-F");
 	}
       else if (!strcmp(argv[argn], "-p"))
 	{
@@ -1220,7 +1260,7 @@ main(int argc, char** argv)
 	{
 	  if (argc < argn + 2)
 	    syntax(argv[0]);
-	  opt_l = to_int(argv[++argn]);
+	  opt_l = to_int_nonneg(argv[++argn], "-l");
 	}
       else if (!strcmp(argv[argn], "-u"))
 	{