Unresolved external symbol during dll linking

ALL,

I have a static link library which exports a class. This class contains a member of std::mutex.

When I link this library to my main project there is no problem, but when I try to link the library to dynamic link library in the same project I'm getting the undefined external symbol.

I double check and all the libraries I link are the same.

Whats weird is that the linker complain about the member and not the class itself.

What could be the issue?

TIA!

P.S. if it matter - I'm working with msvc 2017 on Windows 8.1.