diff --git a/firedrake_rtree/__init__.py b/firedrake_rtree/__init__.py
index 4866000..9f73d75 100644
--- a/firedrake_rtree/__init__.py
+++ b/firedrake_rtree/__init__.py
@@ -18,4 +18,5 @@ def get_lib() -> str:
 
 def get_lib_filename() -> str:
     """Return the path to the rtree-capi shared object."""
-    return os.path.join(get_lib(), f"firedrake_rtree{sysconfig.get_config_var("EXT_SUFFIX")}")
+    suffix = sysconfig.get_config_var("EXT_SUFFIX")
+    return os.path.join(get_lib(), f"firedrake_rtree{suffix}")