diff --git a/internal/configuration/sources/env/helpers_test.go b/internal/configuration/sources/env/helpers_test.go
new file mode 100644
index 00000000..9c60657f
--- /dev/null
+++ b/internal/configuration/sources/env/helpers_test.go
@@ -0,0 +1,22 @@
+package env
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// setTestEnv is used to set environment variables in
+// parallel tests.
+func setTestEnv(t *testing.T, key, value string) {
+	t.Helper()
+	existing := os.Getenv(key)
+	err := os.Setenv(key, value) //nolint:tenv
+	t.Cleanup(func() {
+		err = os.Setenv(key, existing)
+		assert.NoError(t, err)
+	})
+	require.NoError(t, err)
+}
diff --git a/internal/configuration/sources/env/system.go b/internal/configuration/sources/env/system.go
index d8139cc5..672f0938 100644
--- a/internal/configuration/sources/env/system.go
+++ b/internal/configuration/sources/env/system.go
@@ -54,7 +54,7 @@ func (r *Reader) readID(key, retroKey string) (
 	if err != nil {
 		return nil, fmt.Errorf("environment variable %s: %w: %s: %s",
 			idEnvKey, ErrSystemIDNotValid, idString, err)
-	} else if idInt < 0 || idInt > 2^16 {
+	} else if idInt < 0 || idInt > 65535 {
 		return nil, fmt.Errorf("environment variable %s: %w: %d: must be between 0 and 65535",
 			idEnvKey, ErrSystemIDNotValid, idInt)
 	}
diff --git a/internal/configuration/sources/env/system_test.go b/internal/configuration/sources/env/system_test.go
new file mode 100644
index 00000000..7d575110
--- /dev/null
+++ b/internal/configuration/sources/env/system_test.go
@@ -0,0 +1,52 @@
+package env
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Reader_readID(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		keyPrefix      string
+		keyValue       string
+		retroKeyPrefix string
+		retroValue     string
+		id             *uint16
+		errWrapped     error
+		errMessage     string
+	}{
+		"id 1000": {
+			keyPrefix:      "ID",
+			keyValue:       "1000",
+			retroKeyPrefix: "RETRO_ID",
+			id:             uint16Ptr(1000),
+		},
+	}
+
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			suffix := t.Name()
+			key := testCase.keyPrefix + suffix
+			retroKey := testCase.retroKeyPrefix + suffix
+
+			setTestEnv(t, key, testCase.keyValue)
+			setTestEnv(t, retroKey, testCase.retroValue)
+
+			reader := &Reader{}
+			id, err := reader.readID(key, retroKey)
+
+			assert.ErrorIs(t, err, testCase.errWrapped)
+			if err != nil {
+				assert.EqualError(t, err, testCase.errMessage)
+			}
+
+			assert.Equal(t, testCase.id, id)
+		})
+	}
+}