diff --git a/keras/engine/topology.py b/keras/engine/topology.py
index a5e147d62e4..82bed08c26e 100644
--- a/keras/engine/topology.py
+++ b/keras/engine/topology.py
@@ -1279,7 +1279,7 @@ def get_output_shape_for(self, input_shape):
                 output_shape = self._output_shape(input_shape)
                 return output_shape
             elif self._output_shape is not None:
-                return (input_shape[0],) + self._output_shape
+                return (input_shape[0],) + tuple(self._output_shape)
             else:
                 # TODO: consider shape auto-inference with TF
                 raise Exception('The Merge layer ' + self.name +