Unhandled error reported for embeded anonymous field methods

Given the following code:

var sb strings.Builder
sb.WriteString("Hello, ")
sb.WriteString("World!")

I get no warnings.

However the following will:

type MyBuilder struct {
   strings.Builder
}

var mb mytypes.MyBuilder
mb.WriteString("Hello, ")
mb.WriteString("World!")

I get two "Unhandled error" warnings for the two "mb.WriteString" calls.

Is this the expected behavior? WriteString always returns nil for error.