为啥以下代码会产生deadlock?

import (
    "fmt"
)

func main() {
    AlterPrint()
}

// POINT: communicate between goroutines by channel

func AlterPrint(){
  letter, number := make(chan bool), make(chan bool)
  letterDone := make(chan bool)
  numberDone := make(chan bool)
  go func() {
    i := 1
    for {
      if i > 28 {
          numberDone <- true
          return
      }
      select{
        case <-number: {
          fmt.Print(i)
          i++
          fmt.Print(i)
          i++
          letter <- true
          break
        }
        default: {
          break
        }
      }

    }
  }()

  go func(){
    i := 'A'
    for {
      if i > 'Z' {
        letterDone <- true
        return
      }
      select{
        case <-letter: {
          fmt.Print(string(i))
          i++
          fmt.Print(string(i))
          i++
          number <- true
          break
        }
        default: {
          break
        }
      }
    }
  }()
  number <- true
  <- letterDone
  <- numberDone
}

I expect the output of "12AB34CD56EF78GH910IJ1112KL1314MN1516OP1718QR1920ST2122UV2324WX2526YZ2728", but the actual output is

goroutine 1 [chan receive]:
main.AlterPrint()
    /tmp/54841538.go:66 +0x183
main.main()
    /tmp/54841538.go:7 +0x14

goroutine 5 [chan send]:
main.AlterPrint.func1(0xc82000c240, 0xc82000c180, 0xc82000c120)
    /tmp/54841538.go:31 +0x25a
created by main.AlterPrint
    /tmp/54841538.go:40 +0xde
exit status 2