diff --git a/usadelndsoc/solver.py b/usadelndsoc/solver.py
index 61d9511dd771a5905b1f73c4726a6f3a8281595d..62f29851cdf66e3c9416816186a18e8fb23dc9d5 100644
--- a/usadelndsoc/solver.py
+++ b/usadelndsoc/solver.py
@@ -187,6 +187,7 @@ class Solver:
         Solver.__global_id += 1
         self._id = Solver.__global_id
         self._core.reset()
+        self._M = None
 
     def _solve(
         self,