Tensorflow Lite - ValueError: Cannot set tensor: Dimension mismatch

This is probably going to be a stupid question but I am new to machine learning and Tensorflow. I am trying to run object detection API on Raspberry Pi using Tensorflow Lite. I am trying to modify my code with the help of this example

https://github.com/freedomtan/tensorflow/blob/deeplab_tflite_python/tensorflow/contrib/lite/examples/python/object_detection.py

This piece of code will detect object from a image. But instead of a image I want to detect object on real time through Pi camera. I tried to modified this code to read input from camera instead of image. Here is my piece of code -

import numpy as np
from tensorflow.contrib.lite.python import interpreter as interpreter_wrapper
import cv2

cap = cv2.VideoCapture(0)
ret, image_np = cap.read()

PATH_TO_MODEL = "ssd_mobilenet_v1_coco.tflite"

interpreter = tf.contrib.lite.Interpreter(model_path=PATH_TO_MODEL)
interpreter.allocate_tensors()

input_details = interpreter.get_input_details()
output_details = interpreter.get_output_details()

while True:
  # NxHxWxC, H:1, W:2
  height = input_details[0]['shape'][1]
  width = input_details[0]['shape'][2]
  ret, image_np = cap.read()

  image_np_expanded = np.expand_dims(image_np, axis=0)
  #if floating_model:
  image_np_expanded = (np.float32(image_np_expanded) - input_mean) / input_std

  #HERE I AM GETTING ERROR
  interpreter.set_tensor(input_details[0]['index'], image_np_expanded)

  if cv2.waitKey(25) & 0xFF == ord('q'):
    cv2.destroyAllWindows()
    break

but I am getting this error -

Traceback (most recent call last):
  File "New_object_detection.py", line 257, in <module>
    interpreter.set_tensor(input_details[0]['index'], image_np_expanded)
  File "/home/saurabh/.local/lib/python3.6/site-packages/tensorflow/contrib/lite/python/interpreter.py", line 151, in set_tensor
    self._interpreter.SetTensor(tensor_index, value)
  File "/home/saurabh/.local/lib/python3.6/site-packages/tensorflow/contrib/lite/python/interpreter_wrapper/tensorflow_wrap_interpreter_wrapper.py", line 133, in SetTensor
    return _tensorflow_wrap_interpreter_wrapper.InterpreterWrapper_SetTensor(self, i, value)
ValueError: Cannot set tensor: Dimension mismatch

Can anyone please tell me how to fix this error or suggest a tutorial for the same?