Can someone help me with this error message?

[gr8lawrence]

I was trying to walk myself through the code by using the sample images dowloaded from the link the original code provided and got this error message when I ran the 'Build model' block in part 02:

TypeError Traceback (most recent call last)
in ()
1 # build model
----> 2 model = utils.model_builder.get_model_3_class(config_vars["crop_size"], config_vars["crop_size"], activation=None)
3 model.summary()
4
5 #loss = "categorical_crossentropy"

~/unet/unet4nuclei/unet4nuclei/utils/model_builder.py in get_model_3_class(dim1, dim2, activation)
87 def get_model_3_class(dim1, dim2, activation="softmax"):
88
---> 89 [x, y] = get_core(dim1, dim2)
90
91 y = keras.layers.Convolution2D(3, 1, 1, **option_dict_conv)(y)

~/unet/unet4nuclei/unet4nuclei/utils/model_builder.py in get_core(dim1, dim2)
52 d = keras.layers.UpSampling2D()(d)
53
---> 54 y = keras.layers.merge([d, c], concat_axis=3, mode="concat")
55
56 e = keras.layers.Convolution2D(256, 3, 3, **option_dict_conv)(y)

TypeError: 'module' object is not callable

I was frustrated by the error and couldn't figure it out myself. I also didn't change any code except for the work directory in the config.py file. It would be very appreciated if someone can indicate what possibly went wrong or was missing!