Having trouble using gperf:Empty input keyword is not allowed
command_options.gperf:

                    
    %{
    #include "command_options.h"
    typedef struct CommandOptionCode CommandOptionCode;
    %}
    struct CommandOption
      {
      const char *Option;
      int OptionCode;
      };
    %%
    +helpverbose, CommandOptionCode::HELPVERBOSE
    +password, CommandOptionCode::PASSWORD
    +nocopyright, CommandOptionCode::NOCOPYRIGHT
    +nolog, CommandOptionCode::NOLOG
    +_64bit, CommandOptionCode::_64BIT


command_options.h:

                    
    #ifndef __COMMANDOPTIONS_H
    #define __COMMANDOPTIONS_H
    struct CommandOptionCode 
      {
      enum 
        {
        HELPVERBOSE = 1,
        PASSWORD = 2,
        NOCOPYRIGHT = 3,
        NOLOG = 4,
        _64BIT = 5
        };
      };
    #endif

When I run:

    gperf  -L C++ -t --output-file=perfecthash.hpp command_options.gperf

Only to get :

> Empty input keyword is not allowed. To
> recognize an empty input keyword, your
> code should check for len == 0 before
> calling the gperf generated lookup
> function.

Version: GNU gperf 3.0.1
Why?
Having trouble using gperf:Empty input keyword is not allowed
===

command_options.gperf:

                    
    %{
    #include "command_options.h"
    typedef struct CommandOptionCode CommandOptionCode;
    %}
    struct CommandOption
      {
      const char *Option;
      int OptionCode;
      };
    %%
    +helpverbose, CommandOptionCode::HELPVERBOSE
    +password, CommandOptionCode::PASSWORD
    +nocopyright, CommandOptionCode::NOCOPYRIGHT
    +nolog, CommandOptionCode::NOLOG
    +_64bit, CommandOptionCode::_64BIT


command_options.h:

                    
    #ifndef __COMMANDOPTIONS_H
    #define __COMMANDOPTIONS_H
    struct CommandOptionCode 
      {
      enum 
        {
        HELPVERBOSE = 1,
        PASSWORD = 2,
        NOCOPYRIGHT = 3,
        NOLOG = 4,
        _64BIT = 5
        };
      };
    #endif

When I run:

    gperf  -L C++ -t --output-file=perfecthash.hpp command_options.gperf

Only to get :

> Empty input keyword is not allowed. To
> recognize an empty input keyword, your
> code should check for len == 0 before
> calling the gperf generated lookup
> function.

Version: GNU gperf 3.0.1
Why?