diff --git a/etcdserver/api/v3rpc/key.go b/etcdserver/api/v3rpc/key.go
index d00cc6645..ae47aaeae 100644
--- a/etcdserver/api/v3rpc/key.go
+++ b/etcdserver/api/v3rpc/key.go
@@ -38,7 +38,7 @@ func (h *handler) Range(ctx context.Context, r *pb.RangeRequest) (*pb.RangeRespo
 
 	resp, err := h.server.V3DemoDo(ctx, pb.InternalRaftRequest{Range: r})
 	if err != nil {
-		err = togRPCError(err)
+		return nil, togRPCError(err)
 	}
 
 	return resp.(*pb.RangeResponse), err
@@ -51,7 +51,7 @@ func (h *handler) Put(ctx context.Context, r *pb.PutRequest) (*pb.PutResponse, e
 
 	resp, err := h.server.V3DemoDo(ctx, pb.InternalRaftRequest{Put: r})
 	if err != nil {
-		err = togRPCError(err)
+		return nil, togRPCError(err)
 	}
 
 	return resp.(*pb.PutResponse), err
@@ -64,7 +64,7 @@ func (h *handler) DeleteRange(ctx context.Context, r *pb.DeleteRangeRequest) (*p
 
 	resp, err := h.server.V3DemoDo(ctx, pb.InternalRaftRequest{DeleteRange: r})
 	if err != nil {
-		err = togRPCError(err)
+		return nil, togRPCError(err)
 	}
 
 	return resp.(*pb.DeleteRangeResponse), err
@@ -77,7 +77,7 @@ func (h *handler) Txn(ctx context.Context, r *pb.TxnRequest) (*pb.TxnResponse, e
 
 	resp, err := h.server.V3DemoDo(ctx, pb.InternalRaftRequest{Txn: r})
 	if err != nil {
-		err = togRPCError(err)
+		return nil, togRPCError(err)
 	}
 
 	return resp.(*pb.TxnResponse), err
@@ -86,7 +86,7 @@ func (h *handler) Txn(ctx context.Context, r *pb.TxnRequest) (*pb.TxnResponse, e
 func (h *handler) Compact(ctx context.Context, r *pb.CompactionRequest) (*pb.CompactionResponse, error) {
 	resp, err := h.server.V3DemoDo(ctx, pb.InternalRaftRequest{Compaction: r})
 	if err != nil {
-		err = togRPCError(err)
+		return nil, togRPCError(err)
 	}
 
 	return resp.(*pb.CompactionResponse), nil
diff --git a/etcdserver/v3demo_server.go b/etcdserver/v3demo_server.go
index c90d4e5db..0a0bc415a 100644
--- a/etcdserver/v3demo_server.go
+++ b/etcdserver/v3demo_server.go
@@ -27,6 +27,11 @@ type V3DemoServer interface {
 	V3DemoDo(ctx context.Context, r pb.InternalRaftRequest) (proto.Message, error)
 }
 
+type applyResult struct {
+	resp proto.Message
+	err  error
+}
+
 func (s *EtcdServer) V3DemoDo(ctx context.Context, r pb.InternalRaftRequest) (proto.Message, error) {
 	r.ID = s.reqIDGen.Next()
 
@@ -40,8 +45,8 @@ func (s *EtcdServer) V3DemoDo(ctx context.Context, r pb.InternalRaftRequest) (pr
 
 	select {
 	case x := <-ch:
-		resp := x.(proto.Message)
-		return resp, nil
+		result := x.(*applyResult)
+		return result.resp, result.err
 	case <-ctx.Done():
 		s.w.Trigger(r.ID, nil) // GC wait
 		return &pb.EmptyResponse{}, ctx.Err()
@@ -51,54 +56,58 @@ func (s *EtcdServer) V3DemoDo(ctx context.Context, r pb.InternalRaftRequest) (pr
 }
 
 func (s *EtcdServer) applyV3Request(r *pb.InternalRaftRequest) interface{} {
+	ar := &applyResult{}
+
 	switch {
 	case r.Range != nil:
-		return applyRange(s.kv, r.Range)
+		ar.resp, ar.err = applyRange(s.kv, r.Range)
 	case r.Put != nil:
-		return applyPut(s.kv, r.Put)
+		ar.resp, ar.err = applyPut(s.kv, r.Put)
 	case r.DeleteRange != nil:
-		return applyDeleteRange(s.kv, r.DeleteRange)
+		ar.resp, ar.err = applyDeleteRange(s.kv, r.DeleteRange)
 	case r.Txn != nil:
-		return applyTxn(s.kv, r.Txn)
+		ar.resp, ar.err = applyTxn(s.kv, r.Txn)
 	case r.Compaction != nil:
-		return applyCompaction(s.kv, r.Compaction)
+		ar.resp, ar.err = applyCompaction(s.kv, r.Compaction)
 	default:
 		panic("not implemented")
 	}
+
+	return ar
 }
 
-func applyPut(kv dstorage.KV, p *pb.PutRequest) *pb.PutResponse {
+func applyPut(kv dstorage.KV, p *pb.PutRequest) (*pb.PutResponse, error) {
 	resp := &pb.PutResponse{}
 	resp.Header = &pb.ResponseHeader{}
 	rev := kv.Put(p.Key, p.Value)
 	resp.Header.Revision = rev
-	return resp
+	return resp, nil
 }
 
-func applyRange(kv dstorage.KV, r *pb.RangeRequest) *pb.RangeResponse {
+func applyRange(kv dstorage.KV, r *pb.RangeRequest) (*pb.RangeResponse, error) {
 	resp := &pb.RangeResponse{}
 	resp.Header = &pb.ResponseHeader{}
 	kvs, rev, err := kv.Range(r.Key, r.RangeEnd, r.Limit, 0)
 	if err != nil {
-		panic("not handled error")
+		return nil, err
 	}
 
 	resp.Header.Revision = rev
 	for i := range kvs {
 		resp.Kvs = append(resp.Kvs, &kvs[i])
 	}
-	return resp
+	return resp, nil
 }
 
-func applyDeleteRange(kv dstorage.KV, dr *pb.DeleteRangeRequest) *pb.DeleteRangeResponse {
+func applyDeleteRange(kv dstorage.KV, dr *pb.DeleteRangeRequest) (*pb.DeleteRangeResponse, error) {
 	resp := &pb.DeleteRangeResponse{}
 	resp.Header = &pb.ResponseHeader{}
 	_, rev := kv.DeleteRange(dr.Key, dr.RangeEnd)
 	resp.Header.Revision = rev
-	return resp
+	return resp, nil
 }
 
-func applyTxn(kv dstorage.KV, rt *pb.TxnRequest) *pb.TxnResponse {
+func applyTxn(kv dstorage.KV, rt *pb.TxnRequest) (*pb.TxnResponse, error) {
 	var revision int64
 
 	ok := true
@@ -108,6 +117,8 @@ func applyTxn(kv dstorage.KV, rt *pb.TxnRequest) *pb.TxnResponse {
 		}
 	}
 
+	// TODO: check potential errors before actually applying anything
+
 	var reqs []*pb.RequestUnion
 	if ok {
 		reqs = rt.Success
@@ -127,29 +138,41 @@ func applyTxn(kv dstorage.KV, rt *pb.TxnRequest) *pb.TxnResponse {
 	txnResp.Header.Revision = revision
 	txnResp.Responses = resps
 	txnResp.Succeeded = ok
-	return txnResp
+	return txnResp, nil
 }
 
-func applyCompaction(kv dstorage.KV, compaction *pb.CompactionRequest) *pb.CompactionResponse {
+func applyCompaction(kv dstorage.KV, compaction *pb.CompactionRequest) (*pb.CompactionResponse, error) {
 	resp := &pb.CompactionResponse{}
 	resp.Header = &pb.ResponseHeader{}
 	err := kv.Compact(compaction.Revision)
 	if err != nil {
-		panic("handle error")
+		return nil, err
 	}
 	// get the current revision. which key to get is not important.
 	_, resp.Header.Revision, _ = kv.Range([]byte("compaction"), nil, 1, 0)
-	return resp
+	return resp, err
 }
 
 func applyUnion(kv dstorage.KV, union *pb.RequestUnion) *pb.ResponseUnion {
 	switch {
 	case union.RequestRange != nil:
-		return &pb.ResponseUnion{ResponseRange: applyRange(kv, union.RequestRange)}
+		resp, err := applyRange(kv, union.RequestRange)
+		if err != nil {
+			panic("unexpected error during txn")
+		}
+		return &pb.ResponseUnion{ResponseRange: resp}
 	case union.RequestPut != nil:
-		return &pb.ResponseUnion{ResponsePut: applyPut(kv, union.RequestPut)}
+		resp, err := applyPut(kv, union.RequestPut)
+		if err != nil {
+			panic("unexpected error during txn")
+		}
+		return &pb.ResponseUnion{ResponsePut: resp}
 	case union.RequestDeleteRange != nil:
-		return &pb.ResponseUnion{ResponseDeleteRange: applyDeleteRange(kv, union.RequestDeleteRange)}
+		resp, err := applyDeleteRange(kv, union.RequestDeleteRange)
+		if err != nil {
+			panic("unexpected error during txn")
+		}
+		return &pb.ResponseUnion{ResponseDeleteRange: resp}
 	default:
 		// empty union
 		return nil