diff --git a/state/execution.go b/state/execution.go
index a36cbfb7..94105331 100644
--- a/state/execution.go
+++ b/state/execution.go
@@ -278,6 +278,10 @@ func updateValidators(currentSet *types.ValidatorSet, abciUpdates []abci.Validat
 
 	// these are tendermint types now
 	for _, valUpdate := range updates {
+		if valUpdate.VotingPower < 0 {
+			return fmt.Errorf("Voting power can't be negative %v", valUpdate)
+		}
+
 		address := valUpdate.Address
 		_, val := currentSet.GetByAddress(address)
 		if val == nil && valUpdate.VotingPower != 0 {
diff --git a/state/execution_test.go b/state/execution_test.go
index 6b8a7e9e..5e0072c3 100644
--- a/state/execution_test.go
+++ b/state/execution_test.go
@@ -201,6 +201,16 @@ func TestUpdateValidators(t *testing.T) {
 			types.NewValidatorSet([]*types.Validator{val1}),
 			true,
 		},
+
+		{
+			"adding a validator with negative power results in error",
+
+			types.NewValidatorSet([]*types.Validator{val1}),
+			[]abci.Validator{{[]byte{}, types.TM2PB.PubKey(pubkey2), -100}},
+
+			types.NewValidatorSet([]*types.Validator{val1}),
+			true,
+		},
 	}
 
 	for _, tc := range testCases {