diff --git a/nilearn/tests/test_niimg_conversions.py b/nilearn/tests/test_niimg_conversions.py
index 00eacb42c3..61b48c1bbd 100644
--- a/nilearn/tests/test_niimg_conversions.py
+++ b/nilearn/tests/test_niimg_conversions.py
@@ -348,7 +348,7 @@ def _check_memory(list_img_3d):
     # measures with memory_profiler.
     mem_offset = b'a' * 100 * 1024 ** 2
     list(_iter_check_niimg(list_img_3d))
-    del mem_offset
+    return mem_offset
 
 
 @with_memory_profiler
diff --git a/nilearn/tests/test_testing.py b/nilearn/tests/test_testing.py
index f5d548a88a..ddcafa0a55 100644
--- a/nilearn/tests/test_testing.py
+++ b/nilearn/tests/test_testing.py
@@ -10,6 +10,7 @@
 def create_object(size):
     """Just create and return an object containing `size` bytes."""
     mem_use = b'a' * size
+    return mem_use
 
 
 @with_memory_profiler