diff --git a/fairscale/nn/data_parallel/fully_sharded_data_parallel.py b/fairscale/nn/data_parallel/fully_sharded_data_parallel.py
index cdd6e6e8c..5584e6d02 100644
--- a/fairscale/nn/data_parallel/fully_sharded_data_parallel.py
+++ b/fairscale/nn/data_parallel/fully_sharded_data_parallel.py
@@ -1821,7 +1821,10 @@ def _post_reduction_hook(self, param: Parameter, reduced_grad: torch.Tensor) ->
                 param._saved_grad_shard.data += reduced_grad.data
             reduced_grad = param._saved_grad_shard.data
         elif (param.grad is None) and self.fp32_reduce_scatter:
-            param.main_grad = reduced_grad.data
+            if getattr(param, "main_grad", None) is not None:
+                param.main_grad.add_(reduced_grad.data)
+            else:
+                param.main_grad = reduced_grad.data
 
         # Optionally move gradients to CPU, typically used if one is running the optimizer on the CPU. Once the full
         # backwards pass completes, we will set `.grad` to the CPU copy.