pasting would not give a valid preprocessing token

[nbecker@fred.net]

../../include/sc_string.h:120:18: warning: pasting would not give a valid preprocessing token

What does this mean?
Here's the offending code:

#define DECL_RELOP(op) \
    bool operator ## op( const char* s ) const; \
    bool operator ## op( const sc_string& s ) const

    DECL_RELOP(==);
    ...