RDataFrame Sum gets confused by std::string

void test() {

   auto rdf = ROOT::RDF::MakeCsvDataFrame("test.csv");

   // The following line gives an error

   std::string oneline = *(rdf.Sum<std::string>("fstring"));

   // The following line works

   std::string oneline = *(rdf.Reduce([](std::string a, std::string b) {return a+b;}, "fstring"));

   std::cout << oneline << std::endl;

}

In file included from input_line_10:1:

/home/jakob/Documents/CERN/ROOT/forest/build-git/test.C:3:16: error: no viable conversion from 'char' to 'std::string' (aka 'basic_string<char>')

   std::string oneline = *(rdf.Sum<std::string>("fstring"));

               ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/lib/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/bits/basic_string.h:509:7: note: candidate constructor not viable: no known conversion from 'char' to 'const char *' for 1st argument; take the address of the argument with &

      basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())

      ^

/usr/lib/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/bits/basic_string.h:437:7: note: candidate constructor not viable: no known conversion from 'char' to 'const std::__cxx11::basic_string<char> &' for 1st argument

      basic_string(const basic_string& __str)

      ^

/usr/lib/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/bits/basic_string.h:531:7: note: candidate constructor not viable: no known conversion from 'char' to 'std::__cxx11::basic_string<char> &&' for 1st argument

      basic_string(basic_string&& __str) noexcept

      ^

/usr/lib/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/bits/basic_string.h:558:7: note: candidate constructor not viable: no known conversion from 'char' to 'initializer_list<char>' for 1st argument

      basic_string(initializer_list<_CharT> __l, const _Alloc& __a = _Alloc())

fstring

"one"

"line"