diff --git a/harmonizer/merge.ts b/harmonizer/merge.ts
index 21c536a..392681e 100644
--- a/harmonizer/merge.ts
+++ b/harmonizer/merge.ts
@@ -18,6 +18,7 @@ import type {
 	ProviderPreferences,
 	ProviderReleaseErrorMap,
 	ProviderReleaseMap,
+	ReleaseGroupType,
 	ResolvableEntity,
 } from './types.ts';
 
@@ -85,6 +86,9 @@ export function mergeRelease(
 	const availableRegions = new Set<CountryCode>();
 	const excludedRegions = new Set<CountryCode>();
 
+	// temporary list of all release group types
+	const releaseGroupTypes = new Array<Iterable<ReleaseGroupType>>();
+
 	orderByPreference(availableProviders, preferredProviders);
 
 	// Phase 1: Clone properties without specific provider preferences
@@ -127,7 +131,7 @@ export function mergeRelease(
 
 		// Merge release group types
 		if (sourceRelease.types) {
-			mergedRelease.types = mergeTypes(mergedRelease.types || [], sourceRelease.types);
+			releaseGroupTypes.push(sourceRelease.types);
 		}
 
 		// combine availabilities
@@ -143,8 +147,9 @@ export function mergeRelease(
 		});
 	}
 
-	// Extend the types with types guessed from titles
-	guessTypesForRelease(mergedRelease);
+	// guess types from titles and merge all release types
+	releaseGroupTypes.push(guessTypesForRelease(mergedRelease));
+	mergedRelease.types = mergeTypes(...releaseGroupTypes);
 
 	// assign temporary sets to the merge target
 	if (availableRegions.size) {
diff --git a/harmonizer/release_types.test.ts b/harmonizer/release_types.test.ts
index 9c7125a..68a8721 100644
--- a/harmonizer/release_types.test.ts
+++ b/harmonizer/release_types.test.ts
@@ -28,8 +28,8 @@ describe('release types', () => {
 
 		passingCases.forEach(([description, release, expected]) => {
 			it(description, () => {
-				guessTypesForRelease(release);
-				assertEquals(release.types, expected);
+				const guessedTypes = guessTypesForRelease(release);
+				assertEquals(new Set(guessedTypes), new Set(expected));
 			});
 		});
 	});
diff --git a/harmonizer/release_types.ts b/harmonizer/release_types.ts
index 98aa3b6..4f0532f 100644
--- a/harmonizer/release_types.ts
+++ b/harmonizer/release_types.ts
@@ -2,13 +2,13 @@ import { HarmonyRelease, HarmonyTrack, ReleaseGroupType } from './types.ts';
 import { primaryTypeIds } from '@kellnerd/musicbrainz/data/release-group';
 
 /** Guess the types for a release from release and track titles. */
-export function guessTypesForRelease(release: HarmonyRelease) {
+export function guessTypesForRelease(release: HarmonyRelease): Iterable<ReleaseGroupType> {
 	let types = new Set(release.types);
 	types = types.union(guessTypesFromTitle(release.title));
 	if (!types.has('Live') && guessLiveRelease(release.media.flatMap((media) => media.tracklist))) {
 		types.add('Live');
 	}
-	release.types = sortTypes(types);
+	return types;
 }
 
 const detectTypesPatterns = [
@@ -79,18 +79,18 @@ export function sortTypes(types: Iterable<ReleaseGroupType>): ReleaseGroupType[]
  *
  * The result is reduced to unique elements with only a single primary type.
  */
-export function mergeTypes(...typeLists: Array<ReleaseGroupType>[]): ReleaseGroupType[] {
+export function mergeTypes(...typeLists: Iterable<ReleaseGroupType>[]): ReleaseGroupType[] {
 	const primaryTypes = new Set<ReleaseGroupType>();
 	const resultTypes = new Set<ReleaseGroupType>();
-	typeLists.forEach((types) => {
-		types.forEach((type) => {
+	for (const types of typeLists) {
+		for (const type of types) {
 			if (isPrimaryType(type)) {
 				primaryTypes.add(type);
 			} else {
 				resultTypes.add(type);
 			}
-		});
-	});
+		}
+	}
 	if (primaryTypes.size) {
 		resultTypes.add(reducePrimaryTypes(Array.from(primaryTypes)));
 	}