On Windows, DMD defaults to -m32, reggae to -m32mscoff (for DMD), and dub to -m64 (if run on a 64-bit Windows host, otherwise -m32mscoff) for compilers with DMD CLI (dmd, gdmd, ldmd2). ;) Windows_DMD assumes a 32-bit MSVC environment (cl.exe etc.) for the tests, so specify the corresponding dub architecture in the reggae cmdline.