panic: interface conversion: interface is nil, not string

[VitaliyPetroff]

Passed tests: 8 of 11

panic: interface conversion: interface is nil, not string

goroutine 1 [running]:
panic(0x24dcc0, 0xc420446040)
/usr/local/go/src/runtime/panic.go:500 +0x1a1
main.printOutput(0xc4202edbc0, 0x8, 0x8, 0xc4202e4ea0, 0x3, 0x4)
/Users/petroff/golang/src/github.com/karolgorecki/goprove/cmd/goprove/main.go:81 +0x353
main.main()
/Users/petroff/golang/src/github.com/karolgorecki/goprove/cmd/goprove/main.go:64 +0x116

Is it normal? May be I do something wrong?
I just tried to run it on my project.

[karolgorecki]

Ok issue fixed

[VitaliyPetroff]

No works fine. Thx