fmt.Println calling Error instead of String()

I have this code

package main
import (
	"fmt"
	"math"
)
type ErrNegativeSqrt float64
func (s ErrNegativeSqrt) String() string {
	return fmt.Sprintf("%f", float64(s))
}
func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprintf("Cannot Sqrt negative number: %v", float64(e))
}
func Sqrt(x float64) (ErrNegativeSqrt, error) {
	if x < 0 {
		e := ErrNegativeSqrt(x)
		return e, e
	} else {
		return ErrNegativeSqrt(math.Sqrt(x)), nil
	}
}
func main() {
	fmt.Println(Sqrt(2))
	fmt.Println(Sqrt(-2))
}

And the output of this code is

> Cannot Sqrt negative number: 1.4142135623730951 <nil>
> Cannot Sqrt negative number: -2 Cannot Sqrt negative number: -2

When I have implemented the Stringer interface for the ErrNegativeSqrt, why is the fmt.Println invoking the Error() method instead of the String() method?

I am new to go, so I might be missing something very obvious.

The documentation states how the value is converted to a string:

> 4. If an operand implements the error interface, the Error method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).
>
> 5. If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

The error interface comes before Stringer.

A more idiomatic way to write the function is:

func Sqrt(x float64) (float64, error) {
  if x < 0 {
    return 0, ErrNegativeSqrt(x)
  }
  return math.Sqrt(x), nil
}