Error parsing RSA public key

I'm trying to read an RSA public key from file with the following code:

keyBytes, err := ioutil.ReadFile("pubkey.pem")
if err != nil {
  log.Fatal(err)
}
block, _ := pem.Decode(keyBytes)
fmt.Printf("block.Type: %s\n", block.Type)
pubkeyInterface, err := x509.ParsePKIXPublicKey(block.Bytes)
pubkey, ok := pubkeyInterface.(*rsa.PublicKey)                                                 
if !ok {
  log.Fatal("Fatal error")
}
cipher, err := rsa.EncryptPKCS1v15(nil, pubkey, []byte(msg))                                   
if err != nil {
  log.Fatal(err)
}

But I got the following error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x463921]

goroutine 1 [running]:
io.ReadAtLeast(0x0, 0x0, 0xc200089002, 0x70, 0x7e, ...)
    /usr/lib/go/src/pkg/io/io.go:284 +0xf1
io.ReadFull(0x0, 0x0, 0xc200089002, 0x70, 0x7e, ...)
    /usr/lib/go/src/pkg/io/io.go:302 +0x6f
crypto/rsa.nonZeroRandomBytes(0xc200089002, 0x70, 0x7e, 0x0, 0x0, ...)
    /usr/lib/go/src/pkg/crypto/rsa/pkcs1v15.go:134 +0x70
crypto/rsa.EncryptPKCS1v15(0x0, 0x0, 0xc20004c550, 0xc20004c560, 0xd, ...)
    /usr/lib/go/src/pkg/crypto/rsa/pkcs1v15.go:35 +0x236
main.encode(0x536630, 0xd, 0x535ad0, 0x9, 0x54f1b0, ...)
    /home/taot/programming/go/encrypt/read_cert.go:28 +0x355
main.main()
    /home/taot/programming/go/encrypt/read_cert.go:12 +0x32

goroutine 2 [syscall]:

goroutine 3 [runnable]:

Seems something is wrong with my type assertion converting interface{} to *rsa.PublicKey, but I didn't get a compile error.

What's the correct way of doing this? Thanks in advance!

Regards,
Terry

The error is because you're passing nil to rsa.EncryptPKCS1v15. As the first parameter it needs an io.Reader from which to read random bytes. You can use, for example, rand.Reader from package crypto/rand.