why initialize reference succeeds with one constructor but not the other

Our code was using something like below, to pass a const string reference:

const char& str = // passed in;
const std::string& sym(str);

Then I need to fix it for take symbol only as the first 5 chars, I did:

const std::string& sym(str, 5);

but this throws me this compiling error:

error: expression list treated as compound expression in initializer [-fpermissive]

Why can I invoke the first constructor, but no the second one?