diff --git a/pybindmagic/__init__.py b/pybindmagic/__init__.py
index d9015e6..13579b0 100644
--- a/pybindmagic/__init__.py
+++ b/pybindmagic/__init__.py
@@ -174,8 +174,9 @@ def cpp(line, cell, *what):
             # otherwise we import all names that don't begin with _
             names = [x for x in mdl.__dict__ if not x.startswith("_")]
     except Exception as e:
-        with open(os.path.join(cmake_path,"CMakeLists.txt"), 'w') as f:
-            f.write(cmake)
+        if cmake_path is not None:
+            with open(os.path.join(cmake_path,"CMakeLists.txt"), 'w') as f:
+                f.write(cmake)
         if os.path.exists(os.path.join(path,f"{name}.cpp")):
             os.remove(os.path.join(path,f"{name}.cpp"))
         raise e