diff --git a/formats/crossref/document.go b/formats/crossref/document.go
index a5cd4074..7786791a 100644
--- a/formats/crossref/document.go
+++ b/formats/crossref/document.go
@@ -239,9 +239,10 @@ func (d *DateField) Date() (t time.Time, err error) {
 // CombinedTitle returns a longish title.
 func (doc *Document) CombinedTitle() string {
 	if len(doc.Title) > 0 {
-		if len(doc.Subtitle) > 0 {
-			return strutil.UnescapeTrim(fmt.Sprintf("%s : %s", strings.Join(doc.Title, " "), strings.Join(doc.Subtitle, " ")))
-		}
+		// TODO: remove this finally; refs. #21429
+		// if len(doc.Subtitle) > 0 {
+		// 	return strutil.UnescapeTrim(fmt.Sprintf("%s : %s", strings.Join(doc.Title, " "), strings.Join(doc.Subtitle, " ")))
+		// }
 		return strutil.UnescapeTrim(strings.Join(doc.Title, " "))
 	}
 	if len(doc.Subtitle) > 0 {
diff --git a/formats/crossref/document_test.go b/formats/crossref/document_test.go
new file mode 100644
index 00000000..adc3a9a0
--- /dev/null
+++ b/formats/crossref/document_test.go
@@ -0,0 +1,45 @@
+package crossref
+
+import "testing"
+
+func TestDocumentCombinedTitle(t *testing.T) {
+	var cases = []struct {
+		doc    *Document
+		result string
+	}{
+		{
+			doc: &Document{
+				Title:    nil,
+				Subtitle: nil,
+			},
+			result: "",
+		},
+		{
+			doc: &Document{
+				Title:    []string{"Hello"},
+				Subtitle: nil,
+			},
+			result: "Hello",
+		},
+		{
+			doc: &Document{
+				Title:    []string{"Hello"},
+				Subtitle: []string{"Sub"},
+			},
+			result: "Hello : Sub",
+		},
+		{
+			doc: &Document{
+				Title:    []string{"Hello", "Sub"},
+				Subtitle: []string{"Sub"},
+			},
+			result: "Hello : Sub",
+		},
+	}
+	for _, c := range cases {
+		result := c.doc.CombinedTitle()
+		if result != c.result {
+			t.Fatalf("got %v, want %v", result, c.result)
+		}
+	}
+}