diff --git a/pylops_mpi/basicoperators/VStack.py b/pylops_mpi/basicoperators/VStack.py
index 96727f7..f869a9a 100644
--- a/pylops_mpi/basicoperators/VStack.py
+++ b/pylops_mpi/basicoperators/VStack.py
@@ -137,7 +137,7 @@ def _rmatvec(self, x: DistributedArray) -> DistributedArray:
         y1 = []
         for iop, oper in enumerate(self.ops):
             y1.append(oper.rmatvec(x.local_array[self.nnops[iop]: self.nnops[iop + 1]]))
-        y1 = ncp.sum(y1, axis=0)
+        y1 = ncp.sum(ncp.vstack(y1), axis=0)
         y[:] = self.base_comm.allreduce(y1, op=MPI.SUM)
         return y