diff --git a/src/buildFFI.py b/src/buildFFI.py
index 32bf4e1..1e4af13 100644
--- a/src/buildFFI.py
+++ b/src/buildFFI.py
@@ -25,6 +25,10 @@ def make_ffi(name, headers, libraries, includes=[], extra_header=""):
     # Define C declarations
     ffi.cdef(header_content)
 
+    ffi.cdef("""
+        extern "Python" void iter_callback(void*, char*, char*);
+    """)
+
     # Set the C source file
     ffi.set_source(name, '''
     #include "nix_api_util.h"
diff --git a/src/nix/store.py b/src/nix/store.py
index c512678..ceec56d 100644
--- a/src/nix/store.py
+++ b/src/nix/store.py
@@ -80,10 +80,9 @@ def build(self, path: StorePath | str) -> dict[str, str]:
         path = self._ensure_store_path(path)
         res = {}
 
-        # todo extern "Python"
-        @ffi.callback("void(void*, char*, char*)")
+        @ffi.def_extern()
         def iter_callback(userdata: CData, key: CData, path: CData) -> None:
             res[ffi.string(key).decode()] = ffi.string(path).decode()
 
-        lib.nix_store_build(self._store, path._path, ffi.NULL, iter_callback)
+        lib.nix_store_build(self._store, path._path, ffi.NULL, lib_unwrapped.iter_callback)
         return res