diff --git a/types/validator_set.go b/types/validator_set.go
index 132957c15..60376a324 100644
--- a/types/validator_set.go
+++ b/types/validator_set.go
@@ -100,9 +100,10 @@ func (valSet *ValidatorSet) GetByAddress(address []byte) (index int, val *Valida
 }
 
 // GetByIndex returns the validator by index.
-// It returns nil values if index >= len(ValidatorSet.Validators)
+// It returns nil values if index < 0 or
+// index >= len(ValidatorSet.Validators)
 func (valSet *ValidatorSet) GetByIndex(index int) (address []byte, val *Validator) {
-	if index >= len(valSet.Validators) {
+	if index < 0 || index >= len(valSet.Validators) {
 		return nil, nil
 	}
 	val = valSet.Validators[index]
diff --git a/types/vote.go b/types/vote.go
index 658415682..d5de6348b 100644
--- a/types/vote.go
+++ b/types/vote.go
@@ -17,6 +17,7 @@ var (
 	ErrVoteInvalidValidatorAddress = errors.New("Invalid validator address")
 	ErrVoteInvalidSignature        = errors.New("Invalid signature")
 	ErrVoteInvalidBlockHash        = errors.New("Invalid block hash")
+	ErrVoteNil                     = errors.New("Nil vote")
 )
 
 type ErrVoteConflictingVotes struct {
diff --git a/types/vote_set.go b/types/vote_set.go
index dcfb0088d..85a839db9 100644
--- a/types/vote_set.go
+++ b/types/vote_set.go
@@ -123,6 +123,7 @@ func (voteSet *VoteSet) Size() int {
 // Conflicting votes return added=*, err=ErrVoteConflictingVotes.
 // NOTE: vote should not be mutated after adding.
 // NOTE: VoteSet must not be nil
+// NOTE: Vote must not be nil
 func (voteSet *VoteSet) AddVote(vote *Vote) (added bool, err error) {
 	if voteSet == nil {
 		cmn.PanicSanity("AddVote() on nil VoteSet")
@@ -135,6 +136,9 @@ func (voteSet *VoteSet) AddVote(vote *Vote) (added bool, err error) {
 
 // NOTE: Validates as much as possible before attempting to verify the signature.
 func (voteSet *VoteSet) addVote(vote *Vote) (added bool, err error) {
+	if vote == nil {
+		return false, ErrVoteNil
+	}
 	valIndex := vote.ValidatorIndex
 	valAddr := vote.ValidatorAddress
 	blockKey := vote.BlockID.Key()