Test code ( https://play.golang.org/p/z__pwPlGxAT ):

package main

import (
	"fmt"
)


func test(i interface{}){
	switch i.(type) {
	case error:
		fmt.Printf("error type\n")
	default:
		fmt.Printf("other type\n")
	}
	fmt.Printf("%v\n", i)
}

func main() {
	
	i1 := struct {
		string
		error
	}{ "a", nil }

	fmt.Println("--------- case 1 ---------")
	test(i1)

	i2 := struct {
		S string
		error
	}{ "a", nil }

	fmt.Println("--------- case 2 ---------")
	test(i2)

	i3 := struct {
		string
		E error
	}{ "a", nil }

	fmt.Println("--------- case 3 ---------")
	test(i3)

}

Output:

--------- case 1 ---------
error type
%!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
--------- case 2 ---------
error type
%!v(PANIC=runtime error: invalid memory address or nil pointer dereference)
--------- case 3 ---------
other type
{a <nil>}

Conclusion

If an anonymous struct with a error field but without give the field a name, the Printf('%v') will process this struct as an error type, but the error field is nil, Printf will call error.Error() to print its value, so we got an runtime error: invalid memory address or nil pointer dereference error message.