Running this with the busted testing framework makes the busted runner hang with 100% cpu load

gistfile1.lua

local M = {
    a=function(self,msg) 
        self:b(msg) 
    end, 
    b=function(self,msg) 
        print(msg) 
    end
}

describe("Description", function()
    it("first test", function()
        local m = mock(M)
        local msg = {}

        m:a(msg) -- remove this line and it doesnt hang

         assert.spy(m.b).was.called_with(m, msg)
    end)

    it("second test", function()
        local s = mock(M)
    end)
end)