diff --git a/docplex/mp/cplex_engine.py b/docplex/mp/cplex_engine.py
index 1af3611..314fe74 100644
--- a/docplex/mp/cplex_engine.py
+++ b/docplex/mp/cplex_engine.py
@@ -21,6 +21,7 @@
 from docplex.mp.constants import ConflictStatus
 from docplex.mp.constr import IndicatorConstraint, RangeConstraint, BinaryConstraint, \
     EquivalenceConstraint
+from docplex.mp.format import ExchangeFormat
 from docplex.mp.progress import ProgressData
 from docplex.mp.qprogress import QProgressData
 from docplex.mp.solution import SolveSolution, SolutionPool
@@ -2547,6 +2548,9 @@ def var_by_index(idx):
         return ConflictRefinerResult(conflicts, refined_by=self.name)
 
     def export(self, out, exchange_format):
+        if exchange_format is not None and not isinstance(exchange_format, ExchangeFormat):
+            self._model.fatal("Not a valid  exchange format: {0}", exchange_format)
+
         self.sync_cplex()
         if is_string(out):
             path = out
@@ -2563,7 +2567,7 @@ def export(self, out, exchange_format):
                 else:
                     raise DOcplexException("CPLEX error in SAV export: {0!s}", cpx_se)
         else:
-            # assume a file-like object
+            # delegate to cplex, assume write and close methods.
             filetype = exchange_format.filetype
             return self._cplex.write_to_stream(out, filetype)
 
diff --git a/docplex/mp/model.py b/docplex/mp/model.py
index 07391e2..709b9d7 100644
--- a/docplex/mp/model.py
+++ b/docplex/mp/model.py
@@ -5494,13 +5494,17 @@ def _resolve_path(self, path_arg, basename_arg, extension):
     def _make_output_path(self, extension, basename, path=None):
         return make_output_path2(self.name, extension, basename, path)
 
+    def _new_lp_printer(self):
+        printer_kwargs = {'full_obj': self._print_full_obj}
+        lp_printer = LPModelPrinter(**printer_kwargs)
+        return lp_printer
+
     def _get_printer(self, format_spec, do_raise=False, silent=False):
         # INTERNAL
-        printer_kwargs = {'full_obj': self._print_full_obj}
         format_ = parse_format(format_spec)
         printer = None
         if format_.name == 'LP':
-            printer = LPModelPrinter(**printer_kwargs)
+            printer = self._new_lp_printer()
         else:
             if do_raise:
                 self.fatal("Unsupported output format: {0!s}", format_spec)
@@ -5648,7 +5652,6 @@ def export_as_savgz(self, path=None, basename=None):
         """
         return self._export_from_cplex(path, basename, format_spec="sav.gz")
 
-
     def _export_from_cplex(self, path=None, basename=None, hide_user_names=False,
                            format_spec="lp"):
         return self._export(path, basename,
@@ -5713,7 +5716,13 @@ def _export_to_stream(self, stream, hide_user_names=False, format_spec="lp"):
             printer.set_mangle_names(hide_user_names)
             printer.printModel(self, stream)
         else:
-            self.__engine.export(stream, format_spec)
+            self.__engine.export(stream, format_)
+
+    def _export_to_lp_stream(self, out_stream, hide_user_names=False):
+        lp_printer = self._new_lp_printer()
+        assert lp_printer
+        lp_printer.set_mangle_names(hide_user_names)
+        lp_printer.printModel(self, out_stream)
 
     def export_to_stream(self, stream, hide_user_names=False, format_spec="lp"):
         """ Export the model to an output stream in LP format.
@@ -5731,7 +5740,9 @@ def export_to_stream(self, stream, hide_user_names=False, format_spec="lp"):
                 and `c1`, `c2`, ,... for constraints. Default is to keep user names.
 
         """
-        self._export_to_stream(stream, hide_user_names, format_spec)
+        if format_spec != "lp":
+            self.fatal("Model.export_to_stream() is only available for LP format, \"{0}\" not supported", format_spec)
+        self._export_to_lp_stream(stream, hide_user_names)
 
     def export_as_lp_string(self, hide_user_names=False):
         """ Exports the model to a string in LP format.
@@ -5746,7 +5757,9 @@ def export_as_lp_string(self, hide_user_names=False):
         Returns:
             A string, containing the model exported in LP format.
         """
-        return self.export_to_string(hide_user_names, "lp")
+        oss = StringIO()
+        self._export_to_lp_stream(oss, hide_user_names)
+        return oss.getvalue()
 
     @property
     def lp_string(self):
@@ -5788,20 +5801,10 @@ def _export_as_cplex_string(self, format_spec):
         self.__engine.export(bs, _format)
         raw_res = bs.getvalue()
         if _format.is_binary:
-            # for b, by in enumerate(raw_res):
-            #     nl = (b % 21 == 20)
-            #     print(f" {by}", end='\n' if nl else '')
-            # print()
             return raw_res
         else:
             return raw_res.decode(self.parameters.read.fileencoding.get())
 
-    def export_to_string(self, hide_user_names=False, format_spec="lp"):
-        # INTERNAL
-        oss = StringIO()
-        self._export_to_stream(oss, hide_user_names, format_spec)
-        return oss.getvalue()
-
     def export_parameters_as_prm(self, path=None, basename=None):
         # path is either a nonempty path string or None
         self._checker.typecheck_string(path, accept_none=True, accept_empty=False)
diff --git a/docplex/mp/tck.py b/docplex/mp/tck.py
index 769a13b..0805452 100644
--- a/docplex/mp/tck.py
+++ b/docplex/mp/tck.py
@@ -487,7 +487,8 @@ def typecheck_string(self, arg, accept_empty=False, accept_none=False, caller=''
         elif not (arg is None and accept_none):
             s_caller = resolve_caller_as_string(caller)
             qualifier = "non-empty " if not accept_empty else ""
-            self.fatal("{0}Expecting a {2}string, got: {1!r}", s_caller, arg, qualifier)
+            s_none = " or None" if accept_none else ""
+            self.fatal("{0}Expecting a {2}string{3}, got: {1!r}", s_caller, arg, qualifier, s_none)
 
     def typecheck_string_seq(self, arg, accept_empty=False, accept_none=False, caller=''):
         checked_strings = list(arg)