title: Keras Concatenate TypeError: __init__() got multiple values for argument 'axis'
body: <p>I am currently trying to recreate the Unet. At the "upconvolution" part where the outputs of two layers needs to be merged I got the mentioned error. (TypeError: <strong>init</strong>() got multiple values for argument 'axis')</p>
  
  <ul>
  <li>Keras Version: 2.0.6</li>
  <li>Tensorflow-gpu: 1.2.1</li>
  </ul>
  
  <p>Code snippet:</p>
  
  <pre><code>import gzip
  import os
  
  from six.moves import urllib
  import tensorflow as tf
  import numpy as np
  
  from keras.models import Sequential, Model
  from keras.layers import Input, Dropout, Flatten, Concatenate
  from keras.layers import Conv2D, MaxPool2D, Conv2DTranspose
  from keras.utils import np_utils
  import keras.callbacks
  
  # Define model architecture
  input1 = Input((X_train.shape[1], X_train.shape[2], 1))
  
  conv1 = Conv2D(64,(3,3), activation='relu', padding='same')(input1)
  conv1 = Dropout(0.2)(conv1)
  conv1 = Conv2D(64,(3,3), activation='relu', padding='same')(conv1)
  pool1 = MaxPool2D(pool_size=(2,2))(conv1)
  
  conv2 = Conv2D(128,(3,3), activation='relu', padding='same')(pool1)
  conv2 = Dropout(0.2)(conv2)
  conv2 = Conv2D(128,(3,3), activation='relu')(conv2)
  pool2 = MaxPool2D(pool_size=(2,2))(conv2)
  
  conv3 = Conv2D(256,(3,3), activation='relu', padding='same')(pool2)
  conv3 = Dropout(0.2)(conv3)
  conv3 = Conv2D(256,(3,3), activation='relu', padding='same')(conv3)
  pool3 = MaxPool2D(pool_size=(2,2))(conv3)
  
  conv4 = Conv2D(512,(3,3), activation='relu', padding='same')(pool3)
  conv4 = Conv2D(512,(3,3), activation='relu', padding='same')(conv4)
  
  up5 = Concatenate([Conv2DTranspose(256, (2,2), strides=(2,2),padding='same')(conv4), conv3], axis=3)
  conv5 = Conv2D(256,(3,3), activation='relu', padding='same')(up5)
  conv5 = Conv2D(256,(3,3), activation='relu', padding='same')(conv5)
  </code></pre>
  
  <p>Detailed error message:</p>
  
  <pre><code>Traceback (most recent call last):
  
  File "&lt;ipython-input-48-d61955511ff9&gt;", line 1, in &lt;module&gt;
  runfile('C:/Users/.../MNIST_Unet_new.py', wdir='C:/Users/.../Documents/KerasTutorials')
  
  File "C:\ProgramData\Anaconda3\envs\tensorflow-gpu\lib\site-packages\spyder\utils\site\sitecustomize.py", line 688, in runfile
  execfile(filename, namespace)
  
  File "C:\ProgramData\Anaconda3\envs\tensorflow-gpu\lib\site-packages\spyder\utils\site\sitecustomize.py", line 101, in execfile
  exec(compile(f.read(), filename, 'exec'), namespace)
  
  File "C:/Users/.../MNIST_Unet_new.py", line 107, in &lt;module&gt;
  up5 = Concatenate([Conv2DTranspose(256, (2,2), strides=(2,2),padding='same')(conv4), conv3], axis=3)
  
  TypeError: __init__() got multiple values for argument 'axis'
  </code></pre>