diff --git a/migration/dirtyrate.c b/migration/dirtyrate.c
index 40e41e793e..ab9e1301f6 100644
--- a/migration/dirtyrate.c
+++ b/migration/dirtyrate.c
@@ -69,9 +69,8 @@ static struct DirtyRateInfo *query_dirty_rate_info(void)
     struct DirtyRateInfo *info = g_malloc0(sizeof(DirtyRateInfo));
 
     if (qatomic_read(&CalculatingState) == DIRTY_RATE_STATUS_MEASURED) {
+        info->has_dirty_rate = true;
         info->dirty_rate = dirty_rate;
-    } else {
-        info->dirty_rate = -1;
     }
 
     info->status = CalculatingState;
diff --git a/qapi/migration.json b/qapi/migration.json
index 7f5e6fd681..974021a5c8 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -1743,10 +1743,8 @@
 #
 # Information about current dirty page rate of vm.
 #
-# @dirty-rate: @dirtyrate describing the dirty page rate of vm
-#              in units of MB/s.
-#              If this field returns '-1', it means querying has not
-#              yet started or completed.
+# @dirty-rate: an estimate of the dirty page rate of the VM in units of
+#              MB/s, present only when estimating the rate has completed.
 #
 # @status: status containing dirtyrate query status includes
 #          'unstarted' or 'measuring' or 'measured'
@@ -1759,7 +1757,7 @@
 #
 ##
 { 'struct': 'DirtyRateInfo',
-  'data': {'dirty-rate': 'int64',
+  'data': {'*dirty-rate': 'int64',
            'status': 'DirtyRateStatus',
            'start-time': 'int64',
            'calc-time': 'int64'} }