Index: tests/natools-s_expressions-enumeration_io-tests.adb
==================================================================
--- tests/natools-s_expressions-enumeration_io-tests.adb
+++ tests/natools-s_expressions-enumeration_io-tests.adb
@@ -29,10 +29,11 @@
    -------------------------
 
    procedure All_Tests (Report : in out NT.Reporter'Class) is
    begin
       Basic_Usage (Report);
+      Invalid_Atom (Report);
    end All_Tests;
 
 
 
    ----------------------
@@ -62,6 +63,39 @@
       end;
    exception
       when Error : others => Test.Report_Exception (Error);
    end Basic_Usage;
 
+
+   procedure Invalid_Atom (Report : in out NT.Reporter'Class) is
+      Test : NT.Test := Report.Item ("Value on invalid atoms");
+      use type Stream_IO.File_Mode;
+   begin
+      Without_Fallback :
+      declare
+         Found : Stream_IO.File_Mode;
+      begin
+         Found := Test_IO.Value (To_Atom ("invalid-atom"));
+         Test.Fail ("Exception expected, but Value returned "
+           & Stream_IO.File_Mode'Image (Found));
+      exception
+         when Constraint_Error => null;
+      end Without_Fallback;
+
+      With_Fallback :
+      declare
+         Expected : constant Stream_IO.File_Mode := Stream_IO.Out_File;
+         Found : constant Stream_IO.File_Mode
+           := Test_IO.Value (To_Atom ("invalid-atom"), Expected);
+      begin
+         if Expected /= Found then
+            Test.Fail ("Test_IO.Value returned "
+              & Stream_IO.File_Mode'Image (Found)
+              & ", expected "
+              & Stream_IO.File_Mode'Image (Expected));
+         end if;
+      end With_Fallback;
+   exception
+      when Error : others => Test.Report_Exception (Error);
+   end Invalid_Atom;
+
 end Natools.S_Expressions.Enumeration_IO.Tests;

Index: tests/natools-s_expressions-enumeration_io-tests.ads
==================================================================
--- tests/natools-s_expressions-enumeration_io-tests.ads
+++ tests/natools-s_expressions-enumeration_io-tests.ads
@@ -22,7 +22,8 @@
    package NT renames Natools.Tests;
 
    procedure All_Tests (Report : in out NT.Reporter'Class);
 
    procedure Basic_Usage (Report : in out NT.Reporter'Class);
+   procedure Invalid_Atom (Report : in out NT.Reporter'Class);
 
 end Natools.S_Expressions.Enumeration_IO.Tests;