Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version devel +1b870077c8 Fri Aug 3 17:21:10 2018 +0000 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

➜ echo go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/stephen/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/stephen/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/stephen/dev/go"
GOTMPDIR=""
GOTOOLDIR="/Users/stephen/dev/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/stephen/dev/echo/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/xs/bs09x7hj2jqb6hq3frfzrmkh0000gp/T/go-build052705807=/tmp/go-build -gno-record-gcc-switches -fno-commo

What did you do?

1. go mod init.
2. add a main.go missing a package line
3. run go fmt

import (
	"net/http"

	"github.com/labstack/echo"
	"github.com/labstack/echo/middleware"
)

func main() {
	// Echo instance
	e := echo.New()

	// Middleware
	e.Use(middleware.Logger())
	e.Use(middleware.Recover())

	// Route => handler
	e.GET("/", func(c echo.Context) error {
		return c.String(http.StatusOK, "Hello, World!\n")
	})

	// Start server
	e.Logger.Fatal(e.Start(":1323"))
}

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

it to yell at me about the package line

What did you see instead?

➜  echo go fmt main.go
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1452c47]

goroutine 1 [running]:
cmd/go/internal/fmtcmd.runFmt(0x194de80, 0xc0000a60b0, 0x1, 0x1)
	/Users/stephen/dev/go/src/cmd/go/internal/fmtcmd/fmt.go:63 +0x1b7
main.main()
	/Users/stephen/dev/go/src/cmd/go/main.go:218 +0x7d4