Mask-RCNN with Keras : Tried to convert 'shape' to a tensor and failed. Error: None values not supported

I am trying to run the Keras implemention of Mask_RCNN in inference mode. It is basically running this code demo.ipynb

But when I run it, I get the following error on model creation :

ValueError: Tried to convert 'shape' to a tensor and failed. Error: None values not supported.

And here is the stacktrace :

Traceback (most recent call last):
  File "/snap/pycharm-community/128/helpers/pydev/pydevd.py", line 1758, in <module>
    main()
  File "/snap/pycharm-community/128/helpers/pydev/pydevd.py", line 1752, in main
    globals = debugger.run(setup['file'], None, None, is_module)
  File "/snap/pycharm-community/128/helpers/pydev/pydevd.py", line 1147, in run
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "/snap/pycharm-community/128/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "[PATH/TO/Mask_RCNN/]/Own_code/Test.py", line 44, in <module>
    model = modellib.MaskRCNN(mode="inference", model_dir=MODEL_DIR, config=config)
  File "[PATH/TO/Mask_RCNN/]/Mask_RCNN/mrcnn/model.py", line 1833, in __init__
    self.keras_model = self.build(mode=mode, config=config)
  File "[PATH/TO/Mask_RCNN/]/Mask_RCNN/mrcnn/model.py", line 2035, in build
    fc_layers_size=config.FPN_CLASSIF_FC_LAYERS_SIZE)
  File "[PATH/TO/Mask_RCNN/]/Mask_RCNN/mrcnn/model.py", line 947, in fpn_classifier_graph
    mrcnn_bbox = KL.Reshape((s[1], num_classes, 4), name="mrcnn_bbox")(x)
  File "[PATH/TO/venv/]lib/python3.6/site-packages/tensorflow/python/keras/engine/base_layer.py", line 554, in __call__
    outputs = self.call(inputs, *args, **kwargs)
  File "[PATH/TO/venv/]lib/python3.6/site-packages/tensorflow/python/keras/layers/core.py", line 439, in call
    (array_ops.shape(inputs)[0],) + self.target_shape)
  File "[PATH/TO/venv/]lib/python3.6/site-packages/tensorflow/python/ops/gen_array_ops.py", line 7179, in reshape
    "Reshape", tensor=tensor, shape=shape, name=name)
  File "[PATH/TO/venv/]lib/python3.6/site-packages/tensorflow/python/framework/op_def_library.py", line 529, in _apply_op_helper
    (input_name, err))

It look like my problem is the same as described here, but there is no real answer to it.

Notes :

• I am running it with Python 3.6 and Tensorflow 1.13.1
• I have edited the Keras imports to use the Keras version embedded in Tensorflow
• The code that I run to have this error correspond to the 3 first blocks of the demo notebook.

EDIT

Here is the code I am running :

# Root directory of the project
ROOT_DIR = os.path.abspath("../")

# Import Mask RCNN
sys.path.append(ROOT_DIR)  # To find local version of the library

# Import COCO config
sys.path.append(os.path.join(ROOT_DIR, "samples/coco/"))  # To find local version

# Directory to save logs and trained model
MODEL_DIR = os.path.join(ROOT_DIR, "logs")

# Local path to trained weights file
COCO_MODEL_PATH = os.path.join(ROOT_DIR, "mask_rcnn_coco.h5")
# Download COCO trained weights from Releases if needed
if not os.path.exists(COCO_MODEL_PATH):
    utils.download_trained_weights(COCO_MODEL_PATH)

# Directory of images to run detection on
IMAGE_DIR = os.path.join(ROOT_DIR, "images")

class InferenceConfig(coco.CocoConfig):
    # Set batch size to 1 since we'll be running inference on
    # one image at a time. Batch size = GPU_COUNT * IMAGES_PER_GPU
    GPU_COUNT = 1
    IMAGES_PER_GPU = 1

config = InferenceConfig()

# Create model object in inference mode.
model = modellib.MaskRCNN(mode="inference", model_dir=MODEL_DIR, config=config)

This is really a copy/paste of the demo.ipynb from the maskRCNN repo. The last line is where the error occur.

from Mask-RCNN with Keras : Tried to convert 'shape' to a tensor and failed. Error: None values not supported