diff --git a/rpc_util_test.go b/rpc_util_test.go
index b2b85c73..d832b126 100644
--- a/rpc_util_test.go
+++ b/rpc_util_test.go
@@ -47,6 +47,8 @@ import (
 	"google.golang.org/grpc/transport"
 )
 
+var _ CallOption = EmptyCallOption{} // ensure EmptyCallOption implements the interface
+
 func TestSimpleParsing(t *testing.T) {
 	bigMsg := bytes.Repeat([]byte{'x'}, 1<<24)
 	for _, test := range []struct {