Keras give error: ValueError: ('Could not interpret activation function identifier:'

Question

Keras gives the error : ValueError: ('Could not interpret activation function identifier:', [, ])

importing the activations:

from keras.activations import relu, elu, linear, sigmoid

Defining grid of parameters:

params = {'lr': [0.001, 0.1],
    'first_neuron':[5,9],
     'hidden_layers':[1,5,10],
     'batch_size': [30,40,50],
     'epochs': [40],
     'dropout': [0,0.2],
     'kernel_initializer': ['normal'],
     'optimizer': [Adam],
     #'loss':[mean_absolute_error],
     'activation':[],
     'last_activation':['linear']

     }

Calling a model :

regression__model = regression_model(X_air_train, y_air_train, X_air_valid, y_air_valid, params)