Keras custom loss function

I am trying to implement my own loss function in Keras for RNN (LSTM). Here is my code.

import sys
sys.path.insert(0, "C:\\Users\\skaul\\AppData\\Local\\Continuum\\Anaconda3\\envs\\tensorflow\\Lib\\site-packages")

import numpy as np
from keras.models import Sequential
from keras.layers import LSTM, Dense
import keras.backend as K

timesteps = 10
data_dim = 5
timesteps = 10
num_classes = 2

# expected input data shape: (batch_size, timesteps, data_dim)
model = Sequential()
model.add(LSTM(32, return_sequences=True,
               input_shape=(timesteps, data_dim)))  # returns a sequence of vectors of dimension 32
model.add(LSTM(32, return_sequences=True))  # returns a sequence of vectors of dimension 32
model.add(LSTM(32))  # return a single vector of dimension 32
model.add(Dense(2, activation='softmax'))

def custom_loss(y_true, y_pred):
    ytrue = K.argmax(y_true, axis = 1) 
    ypred = K.argmax(y_pred, axis = 1)

    true1 = ytrue
    pred1 = ypred
    pred0 = ypred - K.cast(K.variable(1),dtype = 'int64')
    pred0 = pred0 * K.cast(K.variable(-1),dtype = 'int64')
    tp = K.sum(true1*pred1) #true positives
    fn = K.sum(true1*pred0) #false negatives

    return K.cast(fn/tp,dtype = 'float32')


model.compile(loss = custom_loss,
              optimizer='adam',
              metrics=['accuracy'])

# Generate dummy training data
x_train = np.random.random((1000, timesteps, data_dim))
y_train = np.random.random((1000, num_classes))

# Generate dummy validation data
x_val = np.random.random((100, timesteps, data_dim))
y_val = np.random.random((100, num_classes))

y_a = np.random.random(y_train.shape)
y_b = np.random.random(y_train.shape)

out1 = K.eval(custom_loss(K.variable(y_a), K.variable(y_b)))
print(out1)

model.fit(x_train, y_train, batch_size=64, epochs=5, validation_data=(x_val, y_val))

      

        
        
        
      

    

I am getting the following error

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-0551e4a8e8ed> in <module>()
     52 print(out1)
     53 
---> 54 model.fit(x_train, y_train, batch_size=64, epochs=5, validation_data=(x_val, y_val))

~\AppData\Local\Continuum\Anaconda3\envs\tensorflow\Lib\site-packages\keras\models.py in fit(self, x, y, batch_size, epochs, verbose, callbacks, validation_split, validation_data, shuffle, class_weight, sample_weight, initial_epoch, **kwargs)
    868                               class_weight=class_weight,
    869                               sample_weight=sample_weight,
--> 870                               initial_epoch=initial_epoch)
    871 
    872     def evaluate(self, x, y, batch_size=32, verbose=1,

~\AppData\Local\Continuum\Anaconda3\envs\tensorflow\Lib\site-packages\keras\engine\training.py in fit(self, x, y, batch_size, epochs, verbose, callbacks, validation_split, validation_data, shuffle, class_weight, sample_weight, initial_epoch, **kwargs)
   1488         else:
   1489             ins = x + y + sample_weights
-> 1490         self._make_train_function()
   1491         f = self.train_function
   1492 

~\AppData\Local\Continuum\Anaconda3\envs\tensorflow\Lib\site-packages\keras\engine\training.py in _make_train_function(self)
   1012                 self._collected_trainable_weights,
   1013                 self.constraints,
-> 1014                 self.total_loss)
   1015             updates = self.updates + training_updates
   1016             # Gets loss and metrics. Updates weights at each call.

~\AppData\Local\Continuum\Anaconda3\envs\tensorflow\Lib\site-packages\keras\optimizers.py in get_updates(self, params, constraints, loss)
    420 
    421         for p, g, m, v in zip(params, grads, ms, vs):
--> 422             m_t = (self.beta_1 * m) + (1. - self.beta_1) * g
    423             v_t = (self.beta_2 * v) + (1. - self.beta_2) * K.square(g)
    424             p_t = p - lr_t * m_t / (K.sqrt(v_t) + self.epsilon)

c:\users\skaul\appdata\local\programs\python\python35\lib\site-packages\tensorflow\python\ops\math_ops.py in binary_op_wrapper(x, y)
    827       if not isinstance(y, sparse_tensor.SparseTensor):
    828         try:
--> 829           y = ops.convert_to_tensor(y, dtype=x.dtype.base_dtype, name="y")
    830         except TypeError:
    831           # If the RHS is not a tensor, it might be a tensor aware object

c:\users\skaul\appdata\local\programs\python\python35\lib\site-packages\tensorflow\python\framework\ops.py in convert_to_tensor(value, dtype, name, preferred_dtype)
    674       name=name,
    675       preferred_dtype=preferred_dtype,
--> 676       as_ref=False)
    677 
    678 

c:\users\skaul\appdata\local\programs\python\python35\lib\site-packages\tensorflow\python\framework\ops.py in internal_convert_to_tensor(value, dtype, name, as_ref, preferred_dtype)
    739 
    740         if ret is None:
--> 741           ret = conversion_func(value, dtype=dtype, name=name, as_ref=as_ref)
    742 
    743         if ret is NotImplemented:

c:\users\skaul\appdata\local\programs\python\python35\lib\site-packages\tensorflow\python\framework\constant_op.py in _constant_tensor_conversion_function(v, dtype, name, as_ref)
    111                                          as_ref=False):
    112   _ = as_ref
--> 113   return constant(v, dtype=dtype, name=name)
    114 
    115 

c:\users\skaul\appdata\local\programs\python\python35\lib\site-packages\tensorflow\python\framework\constant_op.py in constant(value, dtype, shape, name, verify_shape)
    100   tensor_value = attr_value_pb2.AttrValue()
    101   tensor_value.tensor.CopyFrom(
--> 102       tensor_util.make_tensor_proto(value, dtype=dtype, shape=shape, verify_shape=verify_shape))
    103   dtype_value = attr_value_pb2.AttrValue(type=tensor_value.tensor.dtype)
    104   const_tensor = g.create_op(

c:\users\skaul\appdata\local\programs\python\python35\lib\site-packages\tensorflow\python\framework\tensor_util.py in make_tensor_proto(values, dtype, shape, verify_shape)
    362   else:
    363     if values is None:
--> 364       raise ValueError("None values not supported.")
    365     # if dtype is provided, forces numpy array to be the type
    366     # provided if possible.

ValueError: None values not supported.

      

        
        
        
      

    

This makes me think that my loss function is returning "No", however my code exits 0.941634

before displaying the error shown above. This comes from the print (out1) statement, which tests the loss function outside the RNN. Any ideas as to what might be wrong?