runtime error: makeslice: len out of range

[clone1018]

2016/08/08 18:45:22 http: panic serving REDACTED:51802: runtime error: makeslice: len out of range
goroutine 2201577 [running]:
net/http.(*conn).serve.func1(0xc8204a9380)
    /usr/lib/go/src/net/http/server.go:1389 +0xc1
panic(0x7ff800, 0xc8206da260)
    /usr/lib/go/src/runtime/panic.go:443 +0x4e9
encoding/json.(*encodeState).marshal.func1(0xc820469860)
    /usr/lib/go/src/encoding/json/encode.go:266 +0x88
panic(0x7ff800, 0xc8206da260)
    /usr/lib/go/src/runtime/panic.go:443 +0x4e9
encoding/json.encodeByteSlice(0xc8200d0000, 0x73d0c0, 0xc82091ec18, 0x197, 0x0)
    /usr/lib/go/src/encoding/json/encode.go:644 +0x17e
encoding/json.(*structEncoder).encode(0xc820100e10, 0xc8200d0000, 0x838e60, 0xc82091ebe0, 0x199, 0x55f00)
    /usr/lib/go/src/encoding/json/encode.go:587 +0x2c4
encoding/json.(*structEncoder).(encoding/json.encode)-fm(0xc8200d0000, 0x838e60, 0xc82091ebe0, 0x199, 0x838e00)
    /usr/lib/go/src/encoding/json/encode.go:601 +0x51
encoding/json.(*arrayEncoder).encode(0xc82048e058, 0xc8200d0000, 0x822f60, 0xc820958800, 0x97, 0x0)
    /usr/lib/go/src/encoding/json/encode.go:690 +0x11b
encoding/json.(*arrayEncoder).(encoding/json.encode)-fm(0xc8200d0000, 0x822f60, 0xc820958800, 0x97, 0x822f00)
    /usr/lib/go/src/encoding/json/encode.go:697 +0x51
encoding/json.(*sliceEncoder).encode(0xc82048e078, 0xc8200d0000, 0x822f60, 0xc820958800, 0x97, 0x0)
    /usr/lib/go/src/encoding/json/encode.go:667 +0xb0
encoding/json.(*sliceEncoder).(encoding/json.encode)-fm(0xc8200d0000, 0x822f60, 0xc820958800, 0x97, 0xc820958800)
    /usr/lib/go/src/encoding/json/encode.go:676 +0x51
encoding/json.(*encodeState).reflectValue(0xc8200d0000, 0x822f60, 0xc820958800, 0x97)
    /usr/lib/go/src/encoding/json/encode.go:301 +0x6b
encoding/json.(*encodeState).marshal(0xc8200d0000, 0x822f60, 0xc820958800, 0x0, 0x0)
    /usr/lib/go/src/encoding/json/encode.go:274 +0xa9
encoding/json.Marshal(0x822f60, 0xc820958800, 0x0, 0x0, 0x0, 0x0, 0x0)
    /usr/lib/go/src/encoding/json/encode.go:139 +0x84
main.(*WebServer).GuessHandler(0xc82000f2a0, 0x7ff5fe906058, 0xc8200ee0d0, 0xc82027c0e0)
    /home/clone1018/GoCode/src/github.com/clone1018/gol-guesser/web.go:38 +0x1db
main.(*WebServer).GuessHandler-fm(0x7ff5fe906058, 0xc8200ee0d0, 0xc82027c0e0)
    /home/clone1018/GoCode/src/github.com/clone1018/gol-guesser/web.go:19 +0x3e
net/http.HandlerFunc.ServeHTTP(0xc82000f370, 0x7ff5fe906058, 0xc8200ee0d0, 0xc82027c0e0)
    /usr/lib/go/src/net/http/server.go:1618 +0x3a
github.com/gorilla/mux.(*Router).ServeHTTP(0xc820078550, 0x7ff5fe906058, 0xc8200ee0d0, 0xc82027c0e0)
    /home/clone1018/GoCode/src/github.com/gorilla/mux/mux.go:107 +0x297
net/http.serverHandler.ServeHTTP(0xc82008e380, 0x7ff5fe906058, 0xc8200ee0d0, 0xc82027c0e0)
    /usr/lib/go/src/net/http/server.go:2081 +0x19e
net/http.(*conn).serve(0xc8204a9380)
    /usr/lib/go/src/net/http/server.go:1472 +0xf2e
created by net/http.(*Server).Serve
    /usr/lib/go/src/net/http/server.go:2137 +0x44e