diff --git a/cmd/icingadb-migrate/convert.go b/cmd/icingadb-migrate/convert.go
index e14746e46..64a6b3498 100644
--- a/cmd/icingadb-migrate/convert.go
+++ b/cmd/icingadb-migrate/convert.go
@@ -150,19 +150,9 @@ func convertCommentRows(
 				},
 				AckHistoryUpserter: history.AckHistoryUpserter{ClearTime: clearTime},
 				SetTime:            setTime,
-				Author: icingadbTypes.String{
-					NullString: sql.NullString{
-						String: row.AuthorName,
-						Valid:  true,
-					},
-				},
-				Comment: icingadbTypes.String{
-					NullString: sql.NullString{
-						String: row.CommentData,
-						Valid:  true,
-					},
-				},
-				ExpireTime: convertTime(row.ExpirationTime, 0),
+				Author:             icingadbTypes.MakeString(row.AuthorName),
+				Comment:            icingadbTypes.MakeString(row.CommentData),
+				ExpireTime:         convertTime(row.ExpirationTime, 0),
 				IsPersistent: icingadbTypes.Bool{
 					Bool:  row.IsPersistent != 0,
 					Valid: true,
@@ -656,13 +646,8 @@ func convertNotificationRows(
 			SendTime:          ts,
 			State:             row.State,
 			PreviousHardState: previousHardState,
-			Text: icingadbTypes.String{
-				NullString: sql.NullString{
-					String: text,
-					Valid:  true,
-				},
-			},
-			UsersNotified: row.ContactsNotified,
+			Text:              icingadbTypes.MakeString(text),
+			UsersNotified:     row.ContactsNotified,
 		})
 
 		allHistory = append(allHistory, &history.HistoryNotification{
diff --git a/pkg/flatten/flatten.go b/pkg/flatten/flatten.go
index 94a6e7ebc..698eff178 100644
--- a/pkg/flatten/flatten.go
+++ b/pkg/flatten/flatten.go
@@ -1,7 +1,6 @@
 package flatten
 
 import (
-	"database/sql"
 	"fmt"
 	"github.com/icinga/icingadb/pkg/types"
 	"strconv"
@@ -32,12 +31,12 @@ func Flatten(value interface{}, prefix string) map[string]types.String {
 			for i, v := range value {
 				flatten(key+"["+strconv.Itoa(i)+"]", v)
 			}
+		case nil:
+			flattened[key] = types.MakeString("null")
+		case float64:
+			flattened[key] = types.MakeString(strconv.FormatFloat(value, 'f', -1, 64))
 		default:
-			val := "null"
-			if value != nil {
-				val = fmt.Sprintf("%v", value)
-			}
-			flattened[key] = types.String{NullString: sql.NullString{String: val, Valid: true}}
+			flattened[key] = types.MakeString(fmt.Sprintf("%v", value))
 		}
 	}
 
diff --git a/pkg/flatten/flatten_test.go b/pkg/flatten/flatten_test.go
new file mode 100644
index 000000000..f84b8d9ec
--- /dev/null
+++ b/pkg/flatten/flatten_test.go
@@ -0,0 +1,45 @@
+package flatten
+
+import (
+	"github.com/icinga/icingadb/pkg/types"
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestFlatten(t *testing.T) {
+	for _, st := range []struct {
+		name   string
+		prefix string
+		value  any
+		output map[string]types.String
+	}{
+		{"nil", "a", nil, map[string]types.String{"a": types.MakeString("null")}},
+		{"bool", "b", true, map[string]types.String{"b": types.MakeString("true")}},
+		{"int", "c", 42, map[string]types.String{"c": types.MakeString("42")}},
+		{"float", "d", 77.7, map[string]types.String{"d": types.MakeString("77.7")}},
+		{"large_float", "e", 1e23, map[string]types.String{"e": types.MakeString("100000000000000000000000")}},
+		{"string", "f", "\x00", map[string]types.String{"f": types.MakeString("\x00")}},
+		{"nil_slice", "g", []any(nil), map[string]types.String{"g": {}}},
+		{"empty_slice", "h", []any{}, map[string]types.String{"h": {}}},
+		{"slice", "i", []any{nil}, map[string]types.String{"i[0]": types.MakeString("null")}},
+		{"nil_map", "j", map[string]any(nil), map[string]types.String{"j": {}}},
+		{"empty_map", "k", map[string]any{}, map[string]types.String{"k": {}}},
+		{"map", "l", map[string]any{" ": nil}, map[string]types.String{"l. ": types.MakeString("null")}},
+		{"map_with_slice", "m", map[string]any{"\t": []any{"ä", "ö", "ü"}, "ß": "s"}, map[string]types.String{
+			"m.\t[0]": types.MakeString("ä"),
+			"m.\t[1]": types.MakeString("ö"),
+			"m.\t[2]": types.MakeString("ü"),
+			"m.ß":     types.MakeString("s"),
+		}},
+		{"slice_with_map", "n", []any{map[string]any{"ä": "a", "ö": "o", "ü": "u"}, "ß"}, map[string]types.String{
+			"n[0].ä": types.MakeString("a"),
+			"n[0].ö": types.MakeString("o"),
+			"n[0].ü": types.MakeString("u"),
+			"n[1]":   types.MakeString("ß"),
+		}},
+	} {
+		t.Run(st.name, func(t *testing.T) {
+			assert.Equal(t, st.output, Flatten(st.value, st.prefix))
+		})
+	}
+}
diff --git a/pkg/icingadb/ha.go b/pkg/icingadb/ha.go
index 74d3b3234..de31f773f 100644
--- a/pkg/icingadb/ha.go
+++ b/pkg/icingadb/ha.go
@@ -186,12 +186,7 @@ func (h *HA) controller() {
 						EntityWithoutChecksum: v1.EntityWithoutChecksum{IdMeta: v1.IdMeta{
 							Id: envId,
 						}},
-						Name: types.String{
-							NullString: sql.NullString{
-								String: envId.String(),
-								Valid:  true,
-							},
-						},
+						Name: types.MakeString(envId.String()),
 					}
 					h.environmentMu.Unlock()
 				}
diff --git a/pkg/types/string.go b/pkg/types/string.go
index f8ead450c..ce2a4ac69 100644
--- a/pkg/types/string.go
+++ b/pkg/types/string.go
@@ -15,6 +15,14 @@ type String struct {
 	sql.NullString
 }
 
+// MakeString constructs a new non-NULL String from s.
+func MakeString(s string) String {
+	return String{sql.NullString{
+		String: s,
+		Valid:  true,
+	}}
+}
+
 // MarshalJSON implements the json.Marshaler interface.
 // Supports JSON null.
 func (s String) MarshalJSON() ([]byte, error) {