diff --git a/bin/autfilt.cc b/bin/autfilt.cc
index 1b57691ac..9b0fe7419 100644
--- a/bin/autfilt.cc
+++ b/bin/autfilt.cc
@@ -647,7 +647,7 @@ parse_opt(int key, char* arg, struct argp_state*)
       jobs.emplace_back(arg, true);
       break;
     case 'n':
-      opt_max_count = to_pos_int(arg);
+      opt_max_count = to_pos_int(arg, "-n/--max-count");
       break;
     case 'u':
       opt->uniq =
@@ -794,15 +794,17 @@ parse_opt(int key, char* arg, struct argp_state*)
       break;
     case OPT_HIGHLIGHT_NONDET:
       {
-        int v = arg ? to_pos_int(arg) : 1;
+        int v = arg ? to_pos_int(arg, "--highlight-nondet") : 1;
         opt_highlight_nondet_edges = opt_highlight_nondet_states = v;
         break;
       }
     case OPT_HIGHLIGHT_NONDET_STATES:
-      opt_highlight_nondet_states = arg ? to_pos_int(arg) : 1;
+      opt_highlight_nondet_states =
+        arg ? to_pos_int(arg, "--highlight-nondet-states") : 1;
       break;
     case OPT_HIGHLIGHT_NONDET_EDGES:
-      opt_highlight_nondet_edges = arg ? to_pos_int(arg) : 1;
+      opt_highlight_nondet_edges =
+        arg ? to_pos_int(arg, "--highlight-nondet-edges") : 1;
       break;
     case OPT_HIGHLIGHT_WORD:
       {
@@ -1021,7 +1023,7 @@ parse_opt(int key, char* arg, struct argp_state*)
       opt_sccs = parse_range(arg, 0, std::numeric_limits<int>::max());
       break;
     case OPT_SEED:
-      opt_seed = to_int(arg);
+      opt_seed = to_int(arg, "--seed");
       break;
     case OPT_SEP_SETS:
       opt_sep_sets = true;
diff --git a/bin/common_conv.cc b/bin/common_conv.cc
index 4320e6b9a..e63969b16 100644
--- a/bin/common_conv.cc
+++ b/bin/common_conv.cc
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2015 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2015, 2018 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -22,50 +22,54 @@
 #include "error.h"
 
 int
-to_int(const char* s)
+to_int(const char* s, const char* where)
 {
   char* endptr;
   int res = strtol(s, &endptr, 10);
   if (*endptr)
-    error(2, 0, "failed to parse '%s' as an integer.", s);
+    error(2, 0, "failed to parse '%s' as an integer (in argument of %s).",
+          s, where);
   return res;
 }
 
 int
-to_pos_int(const char* s)
+to_pos_int(const char* s, const char* where)
 {
-  int res = to_int(s);
+  int res = to_int(s, where);
   if (res < 0)
-    error(2, 0, "%d is not positive", res);
+    error(2, 0, "%d is not positive (in argument of %s)", res, where);
   return res;
 }
 
 unsigned
-to_unsigned (const char *s)
+to_unsigned (const char *s, const char* where)
 {
   char* endptr;
   unsigned res = strtoul(s, &endptr, 10);
   if (*endptr)
-    error(2, 0, "failed to parse '%s' as an unsigned integer.", s);
+    error(2, 0,
+          "failed to parse '%s' as an unsigned integer (in argument of %s).",
+          s, where);
   return res;
 }
 
 float
-to_float(const char* s)
+to_float(const char* s, const char* where)
 {
   char* endptr;
   float res = strtof(s, &endptr);
   if (*endptr)
-    error(2, 0, "failed to parse '%s' as a float.", s);
+    error(2, 0, "failed to parse '%s' as a float (in argument of %s)",
+          s, where);
   return res;
 }
 
 float
-to_probability(const char* s)
+to_probability(const char* s, const char* where)
 {
-  float res = to_float(s);
+  float res = to_float(s, where);
   if (res < 0.0 || res > 1.0)
-    error(2, 0, "%f is not between 0 and 1.", res);
+    error(2, 0, "%f is not between 0 and 1 (in argument of %s).", res, where);
   return res;
 }
 
diff --git a/bin/common_conv.hh b/bin/common_conv.hh
index a41894539..617e97e23 100644
--- a/bin/common_conv.hh
+++ b/bin/common_conv.hh
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2015 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2015, 2018 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -22,11 +22,11 @@
 #include "common_sys.hh"
 #include <spot/twa/twagraph.hh>
 
-int to_int(const char* s);
-int to_pos_int(const char* s);
-unsigned to_unsigned (const char *s);
-float to_float(const char* s);
-float to_probability(const char* s);
+int to_int(const char* s, const char* where);
+int to_pos_int(const char* s, const char* where);
+unsigned to_unsigned (const char *s, const char* where);
+float to_float(const char* s, const char* where);
+float to_probability(const char* s, const char* where);
 
 // Parse the comma or space seperate string of numbers.
 std::vector<long> to_longs(const char* s);
diff --git a/bin/common_trans.cc b/bin/common_trans.cc
index dfd5ab2b9..a0b21e945 100644
--- a/bin/common_trans.cc
+++ b/bin/common_trans.cc
@@ -802,7 +802,7 @@ static int parse_opt_trans(int key, char* arg, struct argp_state*)
       tools_push_trans(arg);
       break;
     case 'T':
-      timeout = to_pos_int(arg);
+      timeout = to_pos_int(arg, "-T/--timeout");
 #if !ENABLE_TIMEOUT
       std::cerr << "warning: setting a timeout is not supported "
                 << "on your platform" << std::endl;
@@ -865,7 +865,7 @@ static int parse_opt_autproc(int key, char* arg, struct argp_state*)
       tools_push_autproc(arg);
       break;
     case 'T':
-      timeout = to_pos_int(arg);
+      timeout = to_pos_int(arg, "-T/--timeout");
 #if !ENABLE_TIMEOUT
       std::cerr << "warning: setting a timeout is not supported "
                 << "on your platform" << std::endl;
diff --git a/bin/ltlcross.cc b/bin/ltlcross.cc
index c5591d0f4..e7195bb75 100644
--- a/bin/ltlcross.cc
+++ b/bin/ltlcross.cc
@@ -448,7 +448,7 @@ parse_opt(int key, char* arg, struct argp_state*)
       csv_output = arg ? arg : "-";
       break;
     case OPT_DENSITY:
-      density = to_probability(arg);
+      density = to_probability(arg, "---density");
       break;
     case OPT_DUPS:
       allow_dups = true;
@@ -472,7 +472,7 @@ parse_opt(int key, char* arg, struct argp_state*)
           products_avg = false;
           ++arg;
         }
-      products = to_pos_int(arg);
+      products = to_pos_int(arg, "--products");
       if (products == 0)
         products_avg = false;
       break;
@@ -490,10 +490,10 @@ parse_opt(int key, char* arg, struct argp_state*)
       tools_push_trans(arg, true);
       break;
     case OPT_SEED:
-      seed = to_pos_int(arg);
+      seed = to_pos_int(arg, "--seed");
       break;
     case OPT_STATES:
-      states = to_pos_int(arg);
+      states = to_pos_int(arg, "--states");
       break;
     case OPT_STOP_ERR:
       stop_on_error = true;
diff --git a/bin/ltldo.cc b/bin/ltldo.cc
index d63e4e4ce..1b7afb48a 100644
--- a/bin/ltldo.cc
+++ b/bin/ltldo.cc
@@ -179,7 +179,7 @@ parse_opt(int key, char* arg, struct argp_state*)
         best_format = arg;
       break;
     case 'n':
-      opt_max_count = to_pos_int(arg);
+      opt_max_count = to_pos_int(arg, "-n/--max-count");
       break;
     case ARGP_KEY_ARG:
       if (arg[0] == '-' && !arg[1])
diff --git a/bin/ltlfilt.cc b/bin/ltlfilt.cc
index 127fc511e..dc507ff83 100644
--- a/bin/ltlfilt.cc
+++ b/bin/ltlfilt.cc
@@ -345,7 +345,7 @@ parse_opt(int key, char* arg, struct argp_state*)
       output_format = count_output;
       break;
     case 'n':
-      opt_max_count = to_pos_int(arg);
+      opt_max_count = to_pos_int(arg, "-n/--max-count");
       break;
     case 'q':
       output_format = quiet_output;
@@ -385,10 +385,10 @@ parse_opt(int key, char* arg, struct argp_state*)
       bsize = parse_range(arg, 0, std::numeric_limits<int>::max());
       break;
     case OPT_BSIZE_MIN:
-      bsize.min = to_int(arg);
+      bsize.min = to_int(arg, "--bsize-min");
       break;
     case OPT_BSIZE_MAX:
-      bsize.max = to_int(arg);
+      bsize.max = to_int(arg, "--bsize-max");
       break;
     case OPT_DEFINE:
       opt->output_define.reset(new output_file(arg ? arg : "-"));
@@ -487,10 +487,10 @@ parse_opt(int key, char* arg, struct argp_state*)
       size = parse_range(arg, 0, std::numeric_limits<int>::max());
       break;
     case OPT_SIZE_MIN:
-      size.min = to_int(arg);
+      size.min = to_int(arg, "--size-min");
       break;
     case OPT_SIZE_MAX:
-      size.max = to_int(arg);
+      size.max = to_int(arg, "--size-max");
       break;
     case OPT_SKIP_ERRORS:
       error_style = skip_errors;
diff --git a/bin/ltlgrind.cc b/bin/ltlgrind.cc
index e6f9a2f75..2e141e863 100644
--- a/bin/ltlgrind.cc
+++ b/bin/ltlgrind.cc
@@ -134,10 +134,10 @@ parse_opt(int key, char* arg, struct argp_state*)
   switch (key)
     {
     case 'm':
-      mutation_nb = to_unsigned(arg);
+      mutation_nb = to_unsigned(arg, "-m/--mutations");
       break;
     case 'n':
-      max_output = to_int(arg);
+      max_output = to_int(arg, "-n/--max-count");
       break;
     case ARGP_KEY_ARG:
       // FIXME: use stat() to distinguish filename from string?
diff --git a/bin/randaut.cc b/bin/randaut.cc
index eeca8a26e..76e17085a 100644
--- a/bin/randaut.cc
+++ b/bin/randaut.cc
@@ -189,10 +189,7 @@ parse_opt(int key, char* arg, struct argp_state* as)
       spot::enable_utf8();
       break;
     case 'a':
-      opt_acc_prob = to_float(arg);
-      if (opt_acc_prob < 0.0 || opt_acc_prob > 1.0)
-        error(2, 0, "probability of acceptance set membership "
-              "should be between 0.0 and 1.0");
+      opt_acc_prob = to_probability(arg, "-a/--acc-probability");
       break;
     case 'A':
       if (looks_like_a_range(arg))
@@ -215,15 +212,13 @@ parse_opt(int key, char* arg, struct argp_state* as)
       ba_wanted = true;
       break;
     case 'e':
-      opt_density = to_float(arg);
-      if (opt_density < 0.0 || opt_density > 1.0)
-        error(2, 0, "density should be between 0.0 and 1.0");
+      opt_density = to_probability(arg, "-e/--density");
       break;
     case 'D':
       opt_deterministic = true;
       break;
     case 'n':
-      opt_automata = to_int(arg);
+      opt_automata = to_int(arg, "-n/--automata");
       break;
     case 'Q':
       opt_states = parse_range(arg);
@@ -243,7 +238,7 @@ parse_opt(int key, char* arg, struct argp_state* as)
       opt_colored = true;
       break;
     case OPT_SEED:
-      opt_seed = to_int(arg);
+      opt_seed = to_int(arg, "--seed");
       opt_seed_str = arg;
       break;
     case ARGP_KEY_ARG:
diff --git a/bin/randltl.cc b/bin/randltl.cc
index 042d9fa74..d35871151 100644
--- a/bin/randltl.cc
+++ b/bin/randltl.cc
@@ -168,7 +168,7 @@ parse_opt(int key, char* arg, struct argp_state* as)
       output = spot::randltlgenerator::LTL;
       break;
     case 'n':
-      opt_formulas = to_int(arg);
+      opt_formulas = to_int(arg, "-n/--formulas");
       break;
     case 'P':
       output = spot::randltlgenerator::PSL;
@@ -196,7 +196,7 @@ parse_opt(int key, char* arg, struct argp_state* as)
       opt_pS = arg;
       break;
     case OPT_SEED:
-      opt_seed = to_int(arg);
+      opt_seed = to_int(arg, "--seed");
       break;
     case OPT_TREE_SIZE:
       opt_tree_size = parse_range(arg);
diff --git a/tests/core/ltlcross3.test b/tests/core/ltlcross3.test
index 2724af33d..16182cbd7 100755
--- a/tests/core/ltlcross3.test
+++ b/tests/core/ltlcross3.test
@@ -296,9 +296,12 @@ test `grep '"aborted",-1' out.csv | wc -l` -eq 4
 test 5 = `wc -l < out.csv`
 check_csv out.csv
 
+# Diagnose empty automata, and make sure %% is correctly replaced by %
+ltlcross --density 2.01 ltl2tgba 2>stderr && exit 1
+grep 'not between 0 and 1' stderr
 
 # Diagnose empty automata, and make sure %% is correctly replaced by %
-run 1 ltlcross ': %f >%O; echo %%>foo' -f a 2>stderr
+run 1 ltlcross --density 0.01 ': %f >%O; echo %%>foo' -f a 2>stderr
 test 2 = `grep -c ':.*empty input' stderr`
 cat foo
 cat >expected<<EOF
diff --git a/tests/core/ltldo.test b/tests/core/ltldo.test
index 204540f05..1d3360046 100755
--- a/tests/core/ltldo.test
+++ b/tests/core/ltldo.test
@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2015-2017 Laboratoire de Recherche et Développement de
+# Copyright (C) 2015-2018 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -161,6 +161,8 @@ grep 'error:.*foo/bar/ltl2baextended -f .*>.*' stderr
 test 2 = `genltl --and-gf=1..4 | ltldo ltl2tgba -n2 | autfilt -c`
 test 3 = `genltl --and-gf=1..2 | ltldo ltl2tgba 'ltl2tgba -s' -n3 | autfilt -c`
 
+ltldo ltl2tgba -n2a 2>err && exit 1
+grep "ltldo: failed to parse '2a'.*-n" err
 
 genltl --rv-counter=9 | ltldo ltl2tgba --stats='
 print("%[up]R + %[uc]R + %[sp]R + %[sc]R - %R\n");
diff --git a/tests/core/randaut.test b/tests/core/randaut.test
index 0951d2742..6707eb050 100755
--- a/tests/core/randaut.test
+++ b/tests/core/randaut.test
@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2014, 2015, 2016, 2017 Laboratoire de Recherche et
+# Copyright (C) 2014, 2015, 2016, 2017, 2018 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -22,6 +22,13 @@
 
 set -e
 
+randaut -e foo 2>err && exit 1
+grep "randaut: failed to parse 'foo' as a float (in argument of -e" err
+randaut -e 3.14 2>err && exit 1
+grep "randaut: 3.1.*is not between 0 and 1 (in argument of -e" err
+randaut -n1a 3 2>err && exit 1
+grep "randaut: failed to parse '1a' as an integer.* -n/--automata)" err
+
 randaut --spin -Q4 a b | ../ikwiad -H -XN - >out
 grep 'States: 4' out
 grep 'AP: 2' out