diff --git a/prednet.py b/prednet.py
index 7faddec..b2e02c7 100755
--- a/prednet.py
+++ b/prednet.py
@@ -119,9 +119,9 @@ def get_output_shape_for(self, input_shape):
             out_nb_row = input_shape[self.row_axis] / 2**self.output_layer_num
             out_nb_col = input_shape[self.column_axis] / 2**self.output_layer_num
             if self.dim_ordering == 'th':
-                out_shape = (-1, out_stack_size, out_nb_row, out_nb_col)
+                out_shape = (out_stack_size, out_nb_row, out_nb_col)
             else:
-                out_shape = (-1, out_nb_row, out_nb_col, out_stack_size)
+                out_shape = (out_nb_row, out_nb_col, out_stack_size)
 
         if self.return_sequences:
             return (input_shape[0], input_shape[1]) + out_shape