diff --git a/types/block.go b/types/block.go
index da5be2b32..34d2cb85d 100644
--- a/types/block.go
+++ b/types/block.go
@@ -108,6 +108,12 @@ func (b *Block) Hash() cmn.HexBytes {
 // MakePartSet returns a PartSet containing parts of a serialized block.
 // This is the form in which the block is gossipped to peers.
 func (b *Block) MakePartSet(partSize int) *PartSet {
+	if b == nil {
+		return nil
+	}
+	b.mtx.Lock()
+	defer b.mtx.Unlock()
+
 	// We prefix the byte length, so that unmarshaling
 	// can easily happen via a reader.
 	bz, err := cdc.MarshalBinary(b)