Cannot deserialize protobuf

Question

I am having trouble de-serializing a protobuf. I am confident that it is serialized properly, as serialization works in all other cases.

Here's a simplified proto file:

syntax = "proto3";

message Request {
  oneof request {
    LoginRequest login = 1;
    SyncRequest sync = 2;
    PushRequest push = 3;
  }
}

message LoginRequest {
  string username = 1;
  string password = 2;
}

message SyncRequest {
}

message PushRequest {
  message Item {
    int64 id = 1;
    string value = 2;
    string date = 3;
  }

  repeated Item items = 1;
}

I read and de-serialize the protobuf in the doPost method of an HttpServlet:

@Override
protected void doPost(final HttpServletRequest httpRequest, final HttpServletResponse httpResponse) throws ServletException, IOException {
  final BufferedReader reader = httpRequest.getReader();
  final byte[] buffer = IOUtils.toByteArray(reader, StandardCharsets.UTF_8);

  final Request request;

  try {
    // TODO: Read from input stream directly.
    request = Request.parseFrom(buffer);
  } catch(final Exception e) {
    throw new ServletException("Failed to deserialize request body.", e);
  }

  // Never reaches this line.
  // ...
}

I get the following error:

11:44:23.199 [qtp1165303897-24] WARN  o.e.j.ee10.servlet.ServletChannel - handleException /api com.google.protobuf.InvalidProtocolBufferException: While parsing a protocol message, the input ended unexpectedly in the middle of a field.  This could mean either that the input has been truncated or that an embedded message misreported its own length.

If I sent Request.login or Request.sync, it successfully de-serializes. The error only occurs for Request.push.