diff --git a/pina/equation/system_equation.py b/pina/equation/system_equation.py
index 9f6d78f..df852dc 100644
--- a/pina/equation/system_equation.py
+++ b/pina/equation/system_equation.py
@@ -17,12 +17,11 @@ class SystemEquation(Equation):
         :param callable equation: A ``torch`` callable equation to
             evaluate the residual
         :param str reduction: Specifies the reduction to apply to the output:
-            ``'none'`` | ``'mean'`` | ``'sum'``. ``'none'``: no reduction 
-            will be applied, ``'mean'``: the sum of the output will be divided
-            by the number of elements in the output, ``'sum'``: the output will
-            be summed. Note: :attr:`size_average` and :attr:`reduce` are in the
-            process of being deprecated, and in the meantime, specifying either of
-            those two args will override :attr:`reduction`. Default: ``'mean'``.
+            ``none`` | ``mean`` | ``sum``  | ``callable``. ``none``: no reduction 
+            will be applied, ``mean``: the sum of the output will be divided
+            by the number of elements in the output, ``sum``: the output will
+            be summed. ``callable`` a callable function to perform reduction,
+            no checks guaranteed. Default: ``mean``.
         """
         check_consistency([list_equation], list)
         check_consistency(reduction, str)
@@ -37,7 +36,7 @@ class SystemEquation(Equation):
             self.reduction = torch.mean
         elif reduction == 'sum':
             self.reduction = torch.sum
-        elif reduction == 'none':
+        elif (reduction == 'none') or callable(reduction):
             self.reduction = reduction
         else:
             raise NotImplementedError('Only mean and sum reductions implemented.')