Keras Concatenate error: Layer concatenate_1 was called with an input that isn’t a symbolic tensor…

Layer concatenate_1 was called with an input that isn't a symbolic tensor. Received type: <class 'tuple'>. Full input:

Original code

x = concatenate((input_1, input_2), axis=1)

Modify code

x = concatenate([input_1, input_2], axis=1)


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