fatal error: unknown caller pc

runtime stack:
runtime.throw(0x133bd5f, 0x11)
	GOROOT/src/runtime/panic.go:1117 +0x72 fp=0xc00060fc30 sp=0xc00060fc00 pc=0x438072
runtime.gentraceback(0x435cbb, 0xc000af7a80, 0x0, 0xc00354f080, 0x0, 0x0, 0x7fffffff, 0xc00060ffa8, 0x0, 0x0, ...)
	GOROOT/src/runtime/traceback.go:261 +0x1a56 fp=0xc00060ff50 sp=0xc00060fc30 pc=0x460a56
runtime.addOneOpenDeferFrame.func1()
	GOROOT/src/runtime/panic.go:717 +0x91 fp=0xc00060ffd0 sp=0xc00060ff50 pc=0x469591
runtime.systemstack(0x0)
	src/runtime/asm_amd64.s:379 +0x66 fp=0xc00060ffd8 sp=0xc00060ffd0 pc=0x470ba6
runtime.mstart()
	GOROOT/src/runtime/proc.go:1246 fp=0xc00060ffe0 sp=0xc00060ffd8 pc=0x43d340