fatal error: unknown caller pc

runtime stack:
runtime.throw({0x135f56a, 0x1fb5920})
	GOROOT/src/runtime/panic.go:1198 +0x71 fp=0xc00091fbd8 sp=0xc00091fba8 pc=0x435631
runtime.gentraceback(0x4428cc, 0xc00091ff78, 0xc0001781a0, 0xc0022aa820, 0x0, 0x0, 0x7fffffff, 0xc00091ff98, 0x400000002, 0x0)
	GOROOT/src/runtime/traceback.go:274 +0x1956 fp=0xc00091ff48 sp=0xc00091fbd8 pc=0x45b8f6
runtime.addOneOpenDeferFrame.func1()
	GOROOT/src/runtime/panic.go:751 +0x6b fp=0xc00091ffc0 sp=0xc00091ff48 pc=0x4341cb
runtime.systemstack()
	src/runtime/asm_amd64.s:383 +0x49 fp=0xc00091ffc8 sp=0xc00091ffc0 pc=0x4672a9