diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index 71ddc92b7b..c0cb72dbe4 100644
--- a/qapi/string-output-visitor.c
+++ b/qapi/string-output-visitor.c
@@ -74,11 +74,27 @@ static StringOutputVisitor *to_sov(Visitor *v)
 
 static void string_output_set(StringOutputVisitor *sov, char *string)
 {
-    if (sov->string) {
-        g_string_free(sov->string, true);
+    switch (sov->list_mode) {
+    case LM_STARTED:
+        sov->list_mode = LM_IN_PROGRESS;
+        /* fall through */
+    case LM_NONE:
+        if (sov->string) {
+            g_string_free(sov->string, true);
+        }
+        sov->string = g_string_new(string);
+        g_free(string);
+        break;
+
+    case LM_IN_PROGRESS:
+    case LM_END:
+        g_string_append(sov->string, ", ");
+        g_string_append(sov->string, string);
+        break;
+
+    default:
+        abort();
     }
-    sov->string = g_string_new(string);
-    g_free(string);
 }
 
 static void string_output_append(StringOutputVisitor *sov, int64_t a)