Unable to compile ‘optional’ in namespace ‘drogon’ does not name a template type drogon::optional<SignatureData> parse(const drogon::HttpRequestPtr& req);

When I try to compile drogon assist I get the following error:
In file included from /home/albaro/drogon-assist/drogon/assist/http_signature.cpp:1:
/home/albaro/drogon-assist/drogon/assist/http_signature.hpp:27:9: error: ‘optional’ in namespace ‘drogon’ does not name a template type
27 | drogon::optional parse(const drogon::HttpRequestPtr& req);
| ^~~~~~~~
/home/albaro/drogon-assist/drogon/assist/http_signature.cpp:117:9: error: ‘optional’ in namespace ‘drogon’ does not name a template type
117 | drogon::optional parse(const drogon::HttpRequestPtr& req)
| ^~~~~~~~
/home/albaro/drogon-assist/drogon/assist/http_signature.cpp: In function ‘bool drassist::http_signature::verify(const HttpRequestPtr&, const Botan::Public_Key&)’:
/home/albaro/drogon-assist/drogon/assist/http_signature.cpp:227:17: error: ‘parse’ was not declared in this scope; did you mean ‘pause’?
227 | auto data = parse(req);
| ^~~~~
| pause
make[2]: *** [CMakeFiles/drogon_assist.dir/build.make:90: CMakeFiles/drogon_assist.dir/drogon/assist/http_signature.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/drogon_assist.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

Please advise,
Albaro