diff --git a/test/cpp/util/grpc_cli.cc b/test/cpp/util/grpc_cli.cc
index 71470e5214f..1a9662694ac 100644
--- a/test/cpp/util/grpc_cli.cc
+++ b/test/cpp/util/grpc_cli.cc
@@ -34,18 +34,21 @@
 /*
   A command line tool to talk to a grpc server.
   Example of talking to grpc interop server:
-  grpc_cli call localhost:50051 UnaryCall src/proto/grpc/testing/test.proto \
-    "response_size:10"  --enable_ssl=false
+  grpc_cli call localhost:50051 UnaryCall "response_size:10" \
+      --proto_file=src/proto/grpc/testing/test.prot --enable_ssl=false
 
   Options:
-    1. --proto_path, if your proto file is not under current working directory,
+    1. --proto_file, use this flag to provide a proto file if the server does
+       does not have the reflection service.
+    2. --proto_path, if your proto file is not under current working directory,
        use this flag to provide a search root. It should work similar to the
-       counterpart in protoc.
-    2. --metadata specifies metadata to be sent to the server, such as:
+       counterpart in protoc. This option is valid only when proto_file is
+       provided.
+    3. --metadata specifies metadata to be sent to the server, such as:
        --metadata="MyHeaderKey1:Value1:MyHeaderKey2:Value2"
-    3. --enable_ssl, whether to use tls.
-    4. --use_auth, if set to true, attach a GoogleDefaultCredentials to the call
-    3. --input_binary_file, a file containing the serialized request. The file
+    4. --enable_ssl, whether to use tls.
+    5. --use_auth, if set to true, attach a GoogleDefaultCredentials to the call
+    6. --input_binary_file, a file containing the serialized request. The file
        can be generated by calling something like:
        protoc --proto_path=src/proto/grpc/testing/ \
          --encode=grpc.testing.SimpleRequest \
@@ -53,7 +56,7 @@
          < input.txt > input.bin
        If this is used and no proto file is provided in the argument list, the
        method string has to be exact in the form of /package.service/method.
-    4. --output_binary_file, a file to write binary format response into, it can
+    7. --output_binary_file, a file to write binary format response into, it can
        be later decoded using protoc:
        protoc --proto_path=src/proto/grpc/testing/ \
        --decode=grpc.testing.SimpleResponse \
diff --git a/test/cpp/util/proto_file_parser.cc b/test/cpp/util/proto_file_parser.cc
index 2ac19858bf7..b1bf0471e15 100644
--- a/test/cpp/util/proto_file_parser.cc
+++ b/test/cpp/util/proto_file_parser.cc
@@ -109,7 +109,10 @@ ProtoFileParser::ProtoFileParser(std::shared_ptr<grpc::Channel> channel,
       desc_pool_(new google::protobuf::DescriptorPool(desc_db_.get())) {
   std::vector<std::string> service_list;
   if (!desc_db_->GetServices(&service_list)) {
-    LogError("Failed to get services");
+    LogError(
+        "Failed to get services from the server, "
+        "it may not have the reflection service.\n"
+        "Please try to use the --proto_file option to provide a proto file.");
   }
   if (has_error_) {
     return;
@@ -177,7 +180,7 @@ grpc::string ProtoFileParser::GetSerializedProto(
     LogError("Failed to parse text format to proto.");
     return "";
   }
-  ok = request_prototype_->SerializeToString(&serialized);
+  ok = msg->SerializeToString(&serialized);
   if (!ok) {
     LogError("Failed to serialize proto.");
     return "";