Index: sem_ch6.adb
===================================================================
--- sem_ch6.adb	(revision 192025)
+++ sem_ch6.adb	(working copy)
@@ -5756,14 +5756,31 @@
 
                declare
                   TSS_Name : constant TSS_Name_Type := Get_TSS_Name (New_Id);
+
                begin
                   if TSS_Name /= TSS_Stream_Read
                     and then TSS_Name /= TSS_Stream_Write
                     and then TSS_Name /= TSS_Stream_Input
                     and then TSS_Name /= TSS_Stream_Output
                   then
-                     Conformance_Error
-                       ("\type of & does not match!", New_Formal);
+                     --  Here we have a definite conformance error. It is worth
+                     --  special casesing the error message for the case of a
+                     --  controlling formal (which excludes null).
+
+                     if Is_Controlling_Formal (New_Formal) then
+                        Error_Msg_Node_2 := Scope (New_Formal);
+                        Conformance_Error
+                         ("\controlling formal& of& excludes null, "
+                           & "declaration must exclude null as well",
+                            New_Formal);
+
+                     --  Normal case (couldn't we give more detail here???)
+
+                     else
+                        Conformance_Error
+                          ("\type of & does not match!", New_Formal);
+                     end if;
+
                      return;
                   end if;
                end;