diff --git a/graphql/graphql.go b/graphql/graphql.go
index 7aa427b45..6417fc9ed 100644
--- a/graphql/graphql.go
+++ b/graphql/graphql.go
@@ -1217,6 +1217,9 @@ func (r *Resolver) Block(ctx context.Context, args struct {
 	Number *Long
 	Hash   *common.Hash
 }) (*Block, error) {
+	if args.Number != nil && args.Hash != nil {
+		return nil, errors.New("only one of number or hash must be specified")
+	}
 	var numberOrHash rpc.BlockNumberOrHash
 	if args.Number != nil {
 		if *args.Number < 0 {