diff --git a/ethosu/vela/tensor.py b/ethosu/vela/tensor.py
index bc0597f..5e97cfe 100644
--- a/ethosu/vela/tensor.py
+++ b/ethosu/vela/tensor.py
@@ -414,27 +414,30 @@
         return rounded_size
 
     def storage_shape_for_sub_purpose(self, sub_purpose, param_a, param_b):
-        shp = list(self.storage_shape)
         if sub_purpose == TensorSubPurpose.DoubleBuffer:
+            shp = list(self.shape)
             assert len(shp) >= 2
             shp[-1] = min(shp[-1], param_a * 2)
-        elif sub_purpose == TensorSubPurpose.RollingBufferX:
-            assert len(shp) == 4
-            shp[0] = 1
-            shp[2] = min(shp[2], param_a)
-        elif sub_purpose == TensorSubPurpose.RollingBufferY:
-            assert len(shp) == 4
-            shp[0] = 1
-            shp[1] = min(shp[1], param_a)
-        elif sub_purpose == TensorSubPurpose.RollingBufferXY:
-            assert len(shp) == 4
-            shp[0] = 1
-            shp[2] = min(shp[2], param_a)
-            shp[1] = min(shp[1], param_b)
-        elif sub_purpose == TensorSubPurpose.Standard:
-            pass
         else:
-            assert 0, "did not expect new sub purpose %s" % (sub_purpose,)
+            shp = list(self.storage_shape)
+            if sub_purpose == TensorSubPurpose.RollingBufferX:
+                assert len(shp) == 4
+                shp[0] = 1
+                shp[2] = min(shp[2], param_a)
+            elif sub_purpose == TensorSubPurpose.RollingBufferY:
+                assert len(shp) == 4
+                shp[0] = 1
+                shp[1] = min(shp[1], param_a)
+            elif sub_purpose == TensorSubPurpose.RollingBufferXY:
+                assert len(shp) == 4
+                shp[0] = 1
+                shp[2] = min(shp[2], param_a)
+                shp[1] = min(shp[1], param_b)
+            elif sub_purpose == TensorSubPurpose.Standard:
+                pass
+            else:
+                assert 0, "did not expect new sub purpose %s" % (sub_purpose,)
+
         return shp
 
     def set_new_sub_purpose(self, sub_purpose, param_a=None, param_b=None):