Loss of MSE always be 0 when keras for topic predict

my input is a 200 dims vector, which is generated by mean of the word2vector of all words of a article, my output is a 50 dims vector,which is generated by the LDA results of a article I want to use mse as the loss function,but the value of the loss always be 0 my code as follows:

model = Sequential()
model.compile(loss='mse', optimizer='rmsprop')


And regardless of the number of epochs, the loss is always 0

First, is your output a one-hot vector of predicted classes? i.e: class one is [1, 0, 0, ...] and class two is [0, 1, 0, 0, ...].