MissingInputError in Keras: Distributing any layer that uses the learning phase throws an exception

keras_distributed_dropout_error.py

from keras.models import Model
from keras.layers import Input, TimeDistributed, Dropout

in1 = Input(batch_shape=(10, 8, 6), name="in1")
# The following Dropout() layer is closely related to the error,
# although the error is thrown at the model._make_predict_function()
# Exchanging Dropout() with other layers that use the learning phase (e.g. GaussianNoise())
# causes the same issues.
# Other layer, like Dense() work perfectly.
out1 = TimeDistributed(Dropout(0.5))(in1)

model = Model(input=in1, output=out1)
model.compile("adam", "mse")
model._make_predict_function()