fatal error: unknown caller pc

runtime stack:
runtime.throw({0x10ca963?, 0x1b8d080?})
	GOROOT/src/runtime/panic.go:992 +0x71 fp=0xc0003bfbf0 sp=0xc0003bfbc0 pc=0x435e51
runtime.gentraceback(0x45ddd2?, 0xc0003bff80?, 0xc0005924e0?, 0xc000d2a1a0?, 0x0, 0x0, 0x7fffffff, 0xc0003bffa0, 0x400000002?, 0x0)
	GOROOT/src/runtime/traceback.go:254 +0x1a36 fp=0xc0003bff50 sp=0xc0003bfbf0 pc=0x45c676
runtime.addOneOpenDeferFrame.func1()
	GOROOT/src/runtime/panic.go:599 +0x6b fp=0xc0003bffc8 sp=0xc0003bff50 pc=0x43500b
runtime.systemstack()
	src/runtime/asm_amd64.s:469 +0x49 fp=0xc0003bffd0 sp=0xc0003bffc8 pc=0x467469