我正在Keras中使用Tensorflow本机优化程序Adam说，代码非常简单。但是，无论何时运行，它总是会失败，并显示以下错误：
File "/usr/local/lib/python2.7/dist-packages/keras/legacy/interfaces.p
y", line 87, in wrapper
return func(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/keras/engine/training.py"
, line 2016, in fit_generator
self._make_train_function()
File "/usr/local/lib/python2.7/dist-packages/keras/engine/training.py"
, line 990, in _make_train_function
loss=self.total_loss)
File "/usr/local/lib/python2.7/dist-packages/keras/legacy/interfaces.p
y", line 87, in wrapper
return func(*args, **kwargs)
File "/usr/local/lib/python2.7/dist-packages/keras/optimizers.py", lin
e 630, in get_updates
grads, global_step=self.iterations)
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/trainin
g/optimizer.py", line 486, in apply_gradients
update_ops.append(processor.update_op(self, grad))
File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/trainin
g/optimizer.py", line 113, in update_op
"Cannot use a constraint function on a sparse variable.")
RuntimeError: Cannot use a constraint function on a sparse variable.