Why am I getting "missing operand" from get_line()?

[John Smith <yoursurrogategod@gmail.com>]

This is my code:

-- run_time_entry.adb:

with Ada.Text_IO.Unbounded_IO;
with Ada.Text_IO;

procedure Run_Time_Entry is
  First_Name : Ada.Strings.Unbounded.Unbounded_String;
  Last_Name  : Ada.Strings.Unbounded.Unbounded_String;
begin
  Ada.Text_IO.Put("Hello.  What is your first name => ");
  First_Name := Ada.Text_IO.Unbounded_IO.Get_Line();
  Ada.Text_IO.Put("What is your last name => ");
  Last_Name  := Ada.Text_IO.Unbounded_IO.Get_Line();

  Ada.Text_IO.Put("Nice to meet you ");
  Ada.Text_IO.Unbounded_IO.Put(First_Name);
  Ada.Text_IO.Put("  ");
  Ada.Text_IO.Unbounded_IO.Put_Line(Last_Name);
end Run_Time_Entry;




This is what happens when I compile it:

> gnatmake -g .\run_time_entry.adb
gcc -c -I.\ -g -I- .\run_time_entry.adb
run_time_entry.adb:11:51: missing operand
run_time_entry.adb:13:51: missing operand
gnatmake: ".\run_time_entry.adb" compilation error

Lines 11 and 13 is where I call Get_Line.  I don't understand, the reference manual (for 2012) says that this function ought to exist and it returns a string.

What am I missing?