fatal error: unknown caller pc

runtime stack:
runtime.throw({0x121ac35, 0x1d74960})
	GOROOT/src/runtime/panic.go:1198 +0x71 fp=0xc000611bd8 sp=0xc000611ba8 pc=0x435631
runtime.gentraceback(0x100c000611f98, 0x4304a5, 0xc000545380, 0xc000699040, 0x0, 0x0, 0x7fffffff, 0xc000611f98, 0x0, 0x0)
	GOROOT/src/runtime/traceback.go:274 +0x1956 fp=0xc000611f48 sp=0xc000611bd8 pc=0x45b8f6
runtime.addOneOpenDeferFrame.func1()
	GOROOT/src/runtime/panic.go:751 +0x6b fp=0xc000611fc0 sp=0xc000611f48 pc=0x4341cb
runtime.systemstack()
	src/runtime/asm_amd64.s:383 +0x49 fp=0xc000611fc8 sp=0xc000611fc0 pc=0x4672a9