diff --git a/CMakeLists.txt b/CMakeLists.txt
index 43c563a9979337afeab298d661551bbf611df67f..de3f2cf536e035103fd554aaf52e06cbf3e8ea75 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -152,7 +152,7 @@ find_python_module(graph_tool   REQUIRED)
 find_python_module(pyopencl     REQUIRED)
 find_python_module(pyfftw       REQUIRED)
 find_python_module(gpyfft       REQUIRED)
-#find_python_module(backports.weakref REQUIRED) # python-backports.weakref
+find_python_module(backports.weakref REQUIRED) # python-backports.weakref
 #find_python_module(backports.functools-lru-cache REQUIRED) # python-backports.functools-lru-cache
 # --- MPI ---
 if(USE_MPI)