diff --git a/SpatialConvolution.lua b/SpatialConvolution.lua
index 71eaa69..e00b648 100644
--- a/SpatialConvolution.lua
+++ b/SpatialConvolution.lua
@@ -23,6 +23,7 @@ function SpatialConvolution:__init(nInputPlane, nOutputPlane,
            'nOutputPlane should be divisible by nGroups')
     self.weight = torch.Tensor(nOutputPlane, nInputPlane/self.groups, kH, kW)
     self.gradWeight = torch.Tensor(nOutputPlane, nInputPlane/self.groups, kH, kW)
+    self.iSize = torch.LongStorage(4):fill(0)
     self:reset()
     -- should nil for serialization, the reset will still work
     self.reset = nil
@@ -99,11 +100,10 @@ function SpatialConvolution:createIODescriptors(input)
         batch = false
     end
     assert(input:dim() == 4 and input:isContiguous());
-    self.iSize = self.iSize or torch.LongStorage(4):fill(0)
     if not self.iDesc or not self.oDesc or
         input:size(1) ~= self.iSize[1] or input:size(2) ~= self.iSize[2]
     or input:size(3) ~= self.iSize[3] or input:size(4) ~= self.iSize[4] then
-        self.iSize = input:size()
+        self.iSize:copy(input:size())
 
         assert(self.nInputPlane == input:size(2), 'input has to contain: '
                    .. self.nInputPlane