diff --git a/miplearn/instance/picklegz.py b/miplearn/instance/picklegz.py
index 7408f02..c5956fa 100644
--- a/miplearn/instance/picklegz.py
+++ b/miplearn/instance/picklegz.py
@@ -126,6 +126,7 @@ class PickleGzInstance(Instance):
     @overrides
     def free(self) -> None:
         self.instance = None  # type: ignore
+        self.samples = None  # type: ignore
         gc.collect()
 
     @overrides
diff --git a/miplearn/solvers/learning.py b/miplearn/solvers/learning.py
index 93c2350..ba85427 100644
--- a/miplearn/solvers/learning.py
+++ b/miplearn/solvers/learning.py
@@ -50,6 +50,7 @@ def _parallel_solve(
             instances[idx],
             discard_output=discard_outputs,
         )
+        instances[idx].free()
         return stats, instances[idx]
     except Exception as e:
         traceback.print_exc()