diff --git a/src/diffpy/srreal/_cleanup.py b/src/diffpy/srreal/_cleanup.py
index 7d7b0b42..dbb41b13 100644
--- a/src/diffpy/srreal/_cleanup.py
+++ b/src/diffpy/srreal/_cleanup.py
@@ -27,6 +27,7 @@ class prototypes is implemented in libdiffpy.  Any Python-extended classes
 """
 
 
+import atexit
 import weakref
 
 
@@ -62,7 +63,7 @@ def add(self, obj):
         return
 
 
-    def __del__(self):
+    def clean(self):
         while self._references:
             wr = self._references.pop()
             obj = wr()
@@ -75,7 +76,8 @@ def __del__(self):
 
 # create singleton instance of the cleanup handler
 _cleanup_handler = _DerivedClassesCleanUpHandler()
-del _DerivedClassesCleanUpHandler
+atexit.register(_cleanup_handler.clean)
 
+del _DerivedClassesCleanUpHandler
 
 # End of file.
diff --git a/src/diffpy/srreal/tests/testpairquantity.py b/src/diffpy/srreal/tests/testpairquantity.py
index c54c27e9..2bf755ab 100644
--- a/src/diffpy/srreal/tests/testpairquantity.py
+++ b/src/diffpy/srreal/tests/testpairquantity.py
@@ -83,7 +83,7 @@ def test_setPairMask_args(self):
         spm = self.pq.setPairMask
         gpm = self.pq.getPairMask
         self.assertRaises(TypeError, spm, 0.0, 0, False)
-        self.assertRaises(TypeError, spm, numpy.complex(0.5), 0, False)
+        self.assertRaises(TypeError, spm, complex(0.5), 0, False)
         self.assertTrue(gpm(0, 0))
         spm(numpy.int32(1), 0, True, others=False)
         self.assertTrue(gpm(0, 1))