keras-team/keras · error · NotImplementedError

We could not automatically infer the shape of the Lambda's o

Error message

We could not automatically infer the shape of the Lambda's output. Please specify the `output_shape` argument for this Lambda layer.

What it means

Error "We could not automatically infer the shape of the Lambda's output. Please specify the `output_shape` argument for this Lambda layer." thrown in keras-team/keras.

Source

Thrown at keras/src/layers/core/lambda_layer.py:95

        # Warning on every invocation will be quite irksome in Eager mode.
        self._already_warned = False

        function_args = inspect.getfullargspec(function).args
        self._fn_expects_training_arg = "training" in function_args
        self._fn_expects_mask_arg = "mask" in function_args

    def compute_output_shape(self, input_shape):
        if self._output_shape is None:
            # Leverage backend shape inference
            try:
                inputs = tree.map_shape_structure(
                    lambda x: backend.KerasTensor(x, dtype=self.compute_dtype),
                    input_shape,
                )
                output_spec = backend.compute_output_spec(self.call, inputs)
                return tree.map_structure(lambda x: x.shape, output_spec)
            except:
                raise NotImplementedError(
                    "We could not automatically infer the shape of "
                    "the Lambda's output. Please specify the `output_shape` "
                    "argument for this Lambda layer."
                )

        if callable(self._output_shape):
            return self._output_shape(input_shape)

        # Output shapes are passed directly and don't include batch dimension.
        batch_size = tree.flatten(input_shape)[0]

        def _add_batch(shape):
            return (batch_size,) + shape

        return tree.map_shape_structure(_add_batch, self._output_shape)

    def call(self, inputs, mask=None, training=None):
        # We must copy for thread safety,

View on GitHub (pinned to 7a34a03db6)

When it happens

Trigger: Thrown at keras/src/layers/core/lambda_layer.py:95 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of keras-team/keras@7a34a03db6 (2026-08-25). Data as JSON: /api/errors/4f5f48a837a79ee8. Report an issue: GitHub.