Go Mod: Cannot Find Module Providing Package

I am creating a go project with version 1.12.1. If I run GOPATH="$(pwd)/vendor:$(pwd)" GOBIN="$(pwd)/bin" go clean I get the following error:

can't load package: package  unknown import path "": cannot find module providing package 

This is only for go clean, go run or go build works fine.

Here is the folder structure of main code:

.
├── Makefile
├── cmd
│   └── server
│       └── main.go
├── go.mod
├── go.sum
└── pkg
    └── storage
        └── mysql
            └── storage.go

Here is how the go.mod file looks like:

module 
go 1.12

require (
     v1.4.1
)

And finally the main.go file:

package main

import (
    "fmt"
    "os"

    ""
)

func main() {
    if err := run(); err != nil {
        fmt.Fprintf(os.Stderr, "%v", err)
        os.Exit(1)
    }
}

func run() error {
    // init storage
    s := mysql.NewStorage()
    // do some other stuff...
}

Any ideas what I am doing wrong?