Pyper: prediction returns None though the model works well in R

I am struggling with the following issue. I trained a model and loaded the trained model in R as follows:

model <- readRDS("C://tests/my_model.rds")
result <- predict(model, trainset, type = "prob")[, 2]

Everything works well and I can get the predictions.

trainset =
   col1    col2    col2
   1       0       1

However, when I try to call the same model using pyper from R, I get this message:

TypeError: object of type 'NoneType' has no len()

In particular, expr = 'model <- readRDS(rmodel); result <- predict(model, rdata, type="prob")[,2]' seems to return None.

This is my code:

import json
import numpy as np
import pandas as pd
from pyper import R

def init():
    global r
    r=R(RCMD="C://Program Files/R/R-3.6.2/bin/x64/R.exe", use_pandas=True)
    rds_path = "C://tests/my_model.rds"
    r.assign("rmodel", rds_path)

def run(raw_data, request_headers):
    data = json.loads(raw_data)["data"]
    cols = json.loads(raw_data)["cols"]
    data = pd.DataFrame(np.array(data), columns = cols)
    r.assign("rdata", data)
    expr = 'model <- readRDS(rmodel); result <- predict(model, rdata, type="prob")[,2]'
    r(expr)
    result = r.get('result')

    print(('{{"RequestId":"{0}", ''"TraceParent":"{1}", ''"NumberOfPredictions":{2}}}'
       ).format(
           request_headers.get("X-Ms-Request-Id", ""),
           request_headers.get("Traceparent", ""),
           len(result)
       ))

    return {"result": result.tolist()}

This is how I call the functions:

init()
test_row = '{"data":[[1,0,1]],"cols":[["col1","col2","col3"]]}'
prediction = run(test_row, {})
print("Test result: ", prediction)

I will really appreciate any help on detecting the issue because the error message is not very informative…

I make experiments in Jupyter Notebook.

Even when I try to substitute expr like this (dummy R code):

expr = 'p <- c(1,2); result <- p'

I get an error:

OSError                                   Traceback (most recent call last)
<ipython-input-97-240ae868eecb> in <module>
      3 test_row  = json.dumps(input_data)
      4 
----> 5 prediction = run(test_row, {})

<ipython-input-96-370705f72e83> in run(raw_data, request_headers)
     23         print(data.head())
     24 
---> 25     r.assign("rdata", data)
     26     expr = 'p <- rdata[1,1:2]; result <- [p]'
     27     r(expr)

~\AppData\Local\Continuum\anaconda3\lib\site-packages\pyper.py in __setitem__(self, obj, val)
    692         if obj.startswith('_'):
    693             raise RError('Leading underscore ("_") is not permitted in R variable names!')
--> 694         self.__call__('%s <- %s' % (obj, Str4R(val)))
    695 
    696     def __delitem__(self, obj):  # to model a dict: "del r['XXX']"

~\AppData\Local\Continuum\anaconda3\lib\site-packages\pyper.py in __call__(self, CMDS, use_try)
    646         rlt = []
    647         if isinstance(CMDS, basestring):  # a single command
--> 648             rlt.append(self.__runOnce(CMDS, use_try=use_try))
    649         else: # should be a list of commands
    650             # for CMD in CMDS:

~\AppData\Local\Continuum\anaconda3\lib\site-packages\pyper.py in __runOnce(self, CMD, use_try)
    622             fn = fn.replace('\\', '/')
    623             CMD = (use_try and 'try({source("%s")})%sfile.remove(%r)%s%s' or '%s%s%s') % (fn, newline, fn, newline, tail_cmd)
--> 624         self.sendAll(self.prog, CMD)
    625         rlt = ''
    626         while not re_tail.search(rlt):

~\AppData\Local\Continuum\anaconda3\lib\site-packages\pyper.py in sendAll(p, s)
    167         p.stdin.write(_mybytes(s))
    168         #os.write(p.stdin.fileno(), s)
--> 169         p.stdin.flush()
    170 
    171     def readLine(p, dump_stdout=False, *a, **b):

OSError: [Errno 22] Invalid argument