diff --git a/test/test_nn.py b/test/test_nn.py
index 69f094c..dbe2c72 100644
--- a/test/test_nn.py
+++ b/test/test_nn.py
@@ -3682,6 +3682,7 @@
         batch_size = 4
         seq_len = 6
         num_directions = 1
+        bad_size = 7  # prime number so that no size can divide it.
 
         def test(input_shape, hidden_shape, mode):
             for input, hidden in get_inputs(input_shape, hidden_shape, mode):
@@ -3691,10 +3692,10 @@
         correct_input_shape = (seq_len, batch_size, input_size)
         correct_hidden_shape = (num_layers * num_directions, batch_size, hidden_size)
 
-        def update_tuple(tup, dim, delta):
-            new_tup = list(tup)
-            new_tup[dim] = delta
-            return tuple(new_tup)
+        def update_shape(shape, dim, new_dim_size):
+            new_shape = list(shape)
+            new_shape[dim] = new_dim_size
+            return tuple(new_shape)
 
         def get_inputs(input_shape, hidden_shape, mode):
             '''returns list( tuple(input, hidden) )
@@ -3714,28 +3715,28 @@
         rnn_modes = ['RNN', 'GRU', 'LSTM']
         for mode in rnn_modes:
             # Incorrect input batch size
-            input_shape = update_tuple(correct_input_shape, 1, -1)
+            input_shape = update_shape(correct_input_shape, 1, bad_size)
             hidden_shape = correct_hidden_shape
             test(input_shape, hidden_shape, mode)
 
             # Incorrect hidden batch size
             input_shape = correct_input_shape
-            hidden_shape = update_tuple(correct_hidden_shape, 1, -1)
+            hidden_shape = update_shape(correct_hidden_shape, 1, bad_size)
             test(input_shape, hidden_shape, mode)
 
             # Incorrect input size
-            input_shape = update_tuple(correct_input_shape, 2, -1)
+            input_shape = update_shape(correct_input_shape, 2, bad_size)
             hidden_shape = correct_hidden_shape
             test(input_shape, hidden_shape, mode)
 
             # Incorrect hidden size
             input_shape = correct_input_shape
-            hidden_shape = update_tuple(correct_hidden_shape, 2, -1)
+            hidden_shape = update_shape(correct_hidden_shape, 2, bad_size)
             test(input_shape, hidden_shape, mode)
 
             # Incorrect hidden[0]
             input_shape = correct_input_shape
-            hidden_shape = update_tuple(correct_hidden_shape, 0, -1)
+            hidden_shape = update_shape(correct_hidden_shape, 0, bad_size)
             test(input_shape, hidden_shape, mode)
 
     def test_rnn_initial_hidden_state(self):