At the Process method iteration is done over the fields slice, but tags variable is being assigned to the tagsGroup slice element with an index which can be out of range. This happens in the case when the tags contain a typo.

For example, this code would fail with panic:

package config

type Config struct {
	Host string `envconfig:"HOST" required:"true"`
	Port string `envconfig:"PORT" required:"true`
}

panic: runtime error: index out of range [1] with length 1