hugolib: Allow untyped nil to be merged in lang.Merge

Fixes #4977

2 files changed, 8 insertions, 0 deletions

diff --git a/hugolib/pages_language_merge.go b/hugolib/pages_language_merge.go
index 6e467c7a0..ef87ac9cb 100644
--- a/hugolib/pages_language_merge.go
+++ b/hugolib/pages_language_merge.go
@@ -53,6 +53,9 @@ func (p1 Pages) MergeByLanguage(p2 Pages) Pages {
 // MergeByLanguageInterface is the generic version of MergeByLanguage. It
 // is here just so it can be called from the tpl package.
 func (p1 Pages) MergeByLanguageInterface(in interface{}) (interface{}, error) {
+	if in == nil {
+		return p1, nil
+	}
 	p2, ok := in.(Pages)
 	if !ok {
 		return nil, fmt.Errorf("%T cannot be merged by language", in)
diff --git a/hugolib/pages_language_merge_test.go b/hugolib/pages_language_merge_test.go
index b7c78836c..8a4688f16 100644
--- a/hugolib/pages_language_merge_test.go
+++ b/hugolib/pages_language_merge_test.go
@@ -79,6 +79,11 @@ func TestMergeLanguages(t *testing.T) {
 	// This looks less ugly in the templates ...
 	mergedNNResources := ri.(resource.ResourcesLanguageMerger).MergeByLanguage(enBundle.Resources)
 	assert.Equal(6, len(mergedNNResources))
+
+	unchanged, err := nnSite.RegularPages.MergeByLanguageInterface(nil)
+	assert.NoError(err)
+	assert.Equal(nnSite.RegularPages, unchanged)
+
 }
 
 func TestMergeLanguagesTemplate(t *testing.T) {