diff --git a/api.go b/api.go
index 40f90f4e..2af58b60 100644
--- a/api.go
+++ b/api.go
@@ -381,7 +381,7 @@ type IndexMetadata struct {
 	IndexMinReaderVersion int
 	IndexTime             time.Time
 	PlainASCII            bool
-	LanguageMap           map[string]uint16
+	LanguageMap           map[string]uint16 // not exported in RPCs
 	ZoektVersion          string
 	ID                    string
 }
diff --git a/eval.go b/eval.go
index 314076ac..8943a8e2 100644
--- a/eval.go
+++ b/eval.go
@@ -99,7 +99,7 @@ func (d *indexData) simplify(in query.Q) query.Q {
 				return r.Set[repo.Name]
 			})
 		case *query.Language:
-			_, has := d.metaData.LanguageMap[r.Language]
+			_, has := d.languageMap[r.Language]
 			if !has && d.metaData.IndexFeatureVersion < 12 {
 				// For index files that haven't been re-indexed by go-enry,
 				// fall back to file-based matching and continue even if this
@@ -270,7 +270,7 @@ nextFileMatch:
 			RepositoryPriority: md.priority,
 			FileName:           string(d.fileName(nextDoc)),
 			Checksum:           d.getChecksum(nextDoc),
-			Language:           d.languageMap[d.getLanguage(nextDoc)],
+			Language:           d.languageMapRev[d.getLanguage(nextDoc)],
 		}
 
 		if s := d.subRepos[nextDoc]; s > 0 {
diff --git a/indexdata.go b/indexdata.go
index c115ffab..8b97175d 100644
--- a/indexdata.go
+++ b/indexdata.go
@@ -86,8 +86,11 @@ type indexData struct {
 	// languages for all the files.
 	languages []byte
 
+	// copy of LanguageMap from Metadata before wiping
+	languageMap map[string]uint16
+
 	// inverse of LanguageMap in metaData
-	languageMap map[uint16]string
+	languageMapRev []string
 
 	repoListEntry []RepoListEntry
 
diff --git a/matchtree.go b/matchtree.go
index 7b0f86f8..172efedd 100644
--- a/matchtree.go
+++ b/matchtree.go
@@ -870,7 +870,7 @@ func (d *indexData) newMatchTree(q query.Q) (matchTree, error) {
 			return &noMatchTree{"const"}, nil
 		}
 	case *query.Language:
-		code, ok := d.metaData.LanguageMap[s.Language]
+		code, ok := d.languageMap[s.Language]
 		if !ok {
 			return &noMatchTree{"lang"}, nil
 		}
diff --git a/merge.go b/merge.go
index b02e2833..83f91c24 100644
--- a/merge.go
+++ b/merge.go
@@ -124,7 +124,7 @@ func merge(ds ...*indexData) (*IndexBuilder, error) {
 				// Content set below since it can return an error
 				// Branches set below since it requires lookups
 				SubRepositoryPath: d.subRepoPaths[repoID][d.subRepos[docID]],
-				Language:          d.languageMap[d.getLanguage(docID)],
+				Language:          d.languageMapRev[d.getLanguage(docID)],
 				// SkipReason not set, will be part of content from original indexer.
 			}
 
diff --git a/read.go b/read.go
index 39965b82..eb3d2bd3 100644
--- a/read.go
+++ b/read.go
@@ -370,10 +370,13 @@ func (r *reader) readIndexData(toc *indexTOC) (*indexData, error) {
 		d.subRepoPaths = append(d.subRepoPaths, keys)
 	}
 
-	d.languageMap = map[uint16]string{}
-	for k, v := range d.metaData.LanguageMap {
-		d.languageMap[v] = k
+	d.languageMap = d.metaData.LanguageMap
+	d.languageMapRev = make([]string, len(d.languageMap))
+	for k, v := range d.languageMap {
+		d.languageMapRev[v] = k
 	}
+	// LanguageMap is entirely useless for readers
+	d.metaData.LanguageMap = nil
 
 	if err := d.verify(); err != nil {
 		return nil, err