diff --git a/SpatialSubtractiveNormalization.lua b/SpatialSubtractiveNormalization.lua
index e2da2c6a2..7fa440267 100644
--- a/SpatialSubtractiveNormalization.lua
+++ b/SpatialSubtractiveNormalization.lua
@@ -63,7 +63,8 @@ end
 function SpatialSubtractiveNormalization:updateOutput(input)   
    -- compute side coefficients
    local dim = input:dim()
-   if input:dim()+1 ~= self.coef:dim() or (input:size(dim) ~= self.coef:size(dim)) or (input:size(dim-1) ~= self.coef:size(dim-1)) then
+   if not self._inpsz or not input:isSize(self._inpsz) then
+      self._inpsz = input:size()
       self.ones = self.ones or input.new()
       self._coef = self._coef or self.coef.new()
       if dim == 4 then