cannot initialize std::unique_ptr class member

Question

I have the following code

Parser.hh

namespace Essembly {
...
public:
    Parser();
private:
    std::unique_ptr<FactoryExpr> factory;
    ...
}

Parser.cc

namespace Essembly {
Parser::Parser(): factory(std::make_unique<FactoryExpr>()) { }
}

and this is the factory FactoryExpr.hh

namespace Essembly {
    class FactoryExpr {
    public:
         FactoryExpr();
         ~FactoryExpr();
    }
}

FactoryExpr.cc

namespace Essembly {
   FactoryExpr::FactoryExpr(){ }
   FactoryExpr::~FactoryExpr() { }

}

Unfortunately, I get the following error

std::__1::unique_ptr<FactoryExpr> Essembly::Parser::factory
no instance of constructor "std::__1::unique_ptr<_Tp, _Dp>::unique_ptr [with _Tp=FactoryExpr, _Dp=std::__1::default_delete<FactoryExpr>]" matches the argument listC/C++(289)
parser.cc(9, 26): argument types are: (std::__1::unique_ptr<Essembly::FactoryExpr, std::__1::default_delete<Essembly::FactoryExpr>>)

I browsed on SO and the MICROSOFT docs, and I think I am doing this correctly. Could someone spot the mistake.