if isSingleTarget && len(candidates) > 0 {
		return Errorf("The following environment variables are unknown: %s",
			strings.Join(candidates.ToSlice(), ", "))
	}

	if !isSingleTarget {
		// Validate other env vars for all targets.
		envVars := candidates.ToSlice()
		for _, envVar := range envVars {
			for _, param := range validKeys {
				pEnvName := getEnvVarName(subSys, Default, param) + Default