diff --git a/petrelic/bn.py b/petrelic/bn.py
index 339c1f8..97a1f48 100644
--- a/petrelic/bn.py
+++ b/petrelic/bn.py
@@ -610,7 +610,7 @@ def __pow__(self, n, modulo=None):
 
         if _C.bn_sign(n.bn) == _C.CONST_RLC_NEG:
             base = base.mod_inverse(modulo)
-            _C.bn_neg(n.bn)
+            _C.bn_neg(n.bn, n.bn)
 
         if _C.bn_is_zero(n.bn) == 1:
             return Bn(1)