diff --git a/internal/blocksync/reactor_test.go b/internal/blocksync/reactor_test.go
index c8a8c79b8..1d4d7d4d6 100644
--- a/internal/blocksync/reactor_test.go
+++ b/internal/blocksync/reactor_test.go
@@ -153,7 +153,7 @@ func (rts *reactorTestSuite) addNode(
 	seenExtCommit := &types.ExtendedCommit{}
 
 	for blockHeight := int64(1); blockHeight <= maxBlockHeight; blockHeight++ {
-		lastExtCommit = seenExtCommit.Copy()
+		lastExtCommit = seenExtCommit.Clone()
 
 		thisBlock := sf.MakeBlock(state, blockHeight, lastExtCommit.StripExtensions())
 		thisParts, err := thisBlock.MakePartSet(types.BlockPartSizeBytes)
diff --git a/types/block.go b/types/block.go
index c81caf213..4f0e22751 100644
--- a/types/block.go
+++ b/types/block.go
@@ -1073,9 +1073,12 @@ type ExtendedCommit struct {
 	bitArray *bits.BitArray
 }
 
-// Copy creates a copy of this extended commit.
-func (ec *ExtendedCommit) Copy() *ExtendedCommit {
+// Clone creates a deep copy of this extended commit.
+func (ec *ExtendedCommit) Clone() *ExtendedCommit {
+	sigs := make([]ExtendedCommitSig, len(ec.ExtendedSignatures))
+	copy(sigs, ec.ExtendedSignatures)
 	ecc := *ec
+	ecc.ExtendedSignatures = sigs
 	return &ecc
 }