diff --git a/consensus/istanbul/backend.go b/consensus/istanbul/backend.go
index e5c37fb2dac4..393ed44a301e 100644
--- a/consensus/istanbul/backend.go
+++ b/consensus/istanbul/backend.go
@@ -46,7 +46,7 @@ type Backend interface {
 
 	// Verify verifies the proposal. If a consensus.ErrFutureBlock error is returned,
 	// the time difference of the proposal and current time is also returned.
-	Verify(Proposal) (error, time.Duration)
+	Verify(Proposal) (time.Duration, error)
 
 	// Sign signs input data with the backend's private key
 	Sign([]byte) ([]byte, error)
diff --git a/consensus/istanbul/backend/backend.go b/consensus/istanbul/backend/backend.go
index 73415c3ffd74..51d6cc2769ec 100644
--- a/consensus/istanbul/backend/backend.go
+++ b/consensus/istanbul/backend/backend.go
@@ -182,23 +182,23 @@ func (sb *backend) EventMux() *event.TypeMux {
 }
 
 // Verify implements istanbul.Backend.Verify
-func (sb *backend) Verify(proposal istanbul.Proposal) (error, time.Duration) {
+func (sb *backend) Verify(proposal istanbul.Proposal) (time.Duration, error) {
 	// Check if the proposal is a valid block
 	block := &types.Block{}
 	block, ok := proposal.(*types.Block)
 	if !ok {
 		sb.logger.Error("Invalid proposal, %v", proposal)
-		return errInvalidProposal, 0
+		return 0, errInvalidProposal
 	}
 	// verify the header of proposed block
 	err := sb.VerifyHeader(sb.chain, block.Header(), false)
 	// ignore errEmptyCommittedSeals error because we don't have the committed seals yet
 	if err == nil || err == errEmptyCommittedSeals {
-		return nil, 0
+		return 0, nil
 	} else if err == consensus.ErrFutureBlock {
-		return consensus.ErrFutureBlock, time.Unix(block.Header().Time.Int64(), 0).Sub(now())
+		return time.Unix(block.Header().Time.Int64(), 0).Sub(now()), consensus.ErrFutureBlock
 	}
-	return err, 0
+	return 0, err
 }
 
 // Sign implements istanbul.Backend.Sign
diff --git a/consensus/istanbul/core/preprepare.go b/consensus/istanbul/core/preprepare.go
index 66edad6e55f4..fa54e6f827e6 100644
--- a/consensus/istanbul/core/preprepare.go
+++ b/consensus/istanbul/core/preprepare.go
@@ -67,7 +67,7 @@ func (c *core) handlePreprepare(msg *message, src istanbul.Validator) error {
 	}
 
 	// Verify the proposal we received
-	if err, duration := c.backend.Verify(preprepare.Proposal); err != nil {
+	if duration, err := c.backend.Verify(preprepare.Proposal); err != nil {
 		logger.Warn("Failed to verify proposal", "err", err, "duration", duration)
 		// if it's a future block, we will handle it again after the duration
 		if err == consensus.ErrFutureBlock {
diff --git a/consensus/istanbul/core/testbackend_test.go b/consensus/istanbul/core/testbackend_test.go
index 3b7a5aecefd1..5c32dec5adfe 100644
--- a/consensus/istanbul/core/testbackend_test.go
+++ b/consensus/istanbul/core/testbackend_test.go
@@ -106,8 +106,8 @@ func (self *testSystemBackend) Commit(proposal istanbul.Proposal, seals [][]byte
 	return nil
 }
 
-func (self *testSystemBackend) Verify(proposal istanbul.Proposal) (error, time.Duration) {
-	return nil, 0
+func (self *testSystemBackend) Verify(proposal istanbul.Proposal) (time.Duration, error) {
+	return 0, nil
 }
 
 func (self *testSystemBackend) Sign(data []byte) ([]byte, error) {