diff --git a/weed/mq/kafka/protocol/joingroup.go b/weed/mq/kafka/protocol/joingroup.go
index c35e74b06..52131167d 100644
--- a/weed/mq/kafka/protocol/joingroup.go
+++ b/weed/mq/kafka/protocol/joingroup.go
@@ -167,8 +167,15 @@ func (h *Handler) handleJoinGroup(connContext *ConnectionContext, correlationID
 		if isNewMember {
 			group.State = consumer.GroupStatePreparingRebalance
 			group.Generation++
+			// Clear prior-generation assignments. The non-leader SyncGroup
+			// path in handleSyncGroup only returns REBALANCE_IN_PROGRESS
+			// when `member.Assignment` is empty; without clearing, a
+			// member rejoining at the new generation would be served its
+			// stale old-generation partitions from the pre-rebalance
+			// state.
 			for _, m := range group.Members {
 				m.State = consumer.MemberStatePending
+				m.Assignment = nil
 			}
 		}
 	case consumer.GroupStateDead:
@@ -946,8 +953,14 @@ func (h *Handler) handleSyncGroup(correlationID uint32, apiVersion uint16, reque
 					request.MemberID, mid)
 				group.State = consumer.GroupStatePreparingRebalance
 				group.Generation++
+				// Clear prior-generation assignments so the non-leader
+				// SyncGroup path (which only returns REBALANCE_IN_PROGRESS
+				// when member.Assignment is empty) doesn't serve stale
+				// partitions between now and the next successful
+				// leader SyncGroup.
 				for _, m := range group.Members {
 					m.State = consumer.MemberStatePending
+					m.Assignment = nil
 				}
 				return h.buildSyncGroupErrorResponse(correlationID, ErrorCodeRebalanceInProgress, apiVersion), nil
 			}