diff --git a/src/dbus_fast/service.pxd b/src/dbus_fast/service.pxd
index 384f54f4..40a2ad3c 100644
--- a/src/dbus_fast/service.pxd
+++ b/src/dbus_fast/service.pxd
@@ -17,6 +17,14 @@ cdef class _Method:
     cdef public SignatureTree in_signature_tree
     cdef public SignatureTree out_signature_tree
 
+
+
+cdef tuple _real_fn_result_to_body(
+    object result,
+    SignatureTree signature_tree,
+    bint replace_fds
+)
+
 cdef class ServiceInterface:
 
     cdef public str name
diff --git a/src/dbus_fast/service.py b/src/dbus_fast/service.py
index 55510fe4..25fe1028 100644
--- a/src/dbus_fast/service.py
+++ b/src/dbus_fast/service.py
@@ -326,6 +326,35 @@ def decorator(fn):
     return decorator
 
 
+def _real_fn_result_to_body(
+    result: Optional[Any],
+    signature_tree: SignatureTree,
+    replace_fds: bool,
+) -> Tuple[List[Any], List[int]]:
+    out_len = len(signature_tree.types)
+    if result is None:
+        final_result = []
+    else:
+        if out_len == 1:
+            final_result = [result]
+        else:
+            result_type = type(result)
+            if result_type is not list and result_type is not tuple:
+                raise SignatureBodyMismatchError(
+                    "Expected signal to return a list or tuple of arguments"
+                )
+            final_result = result
+
+    if out_len != len(final_result):
+        raise SignatureBodyMismatchError(
+            f"Signature and function return mismatch, expected {len(signature_tree.types)} arguments but got {len(result)}"
+        )
+
+    if not replace_fds:
+        return final_result, []
+    return replace_fds_with_idx(signature_tree, final_result)
+
+
 class ServiceInterface:
     """An abstract class that can be extended by the user to define DBus services.
 
@@ -526,28 +555,7 @@ def _fn_result_to_body(
         signature_tree: SignatureTree,
         replace_fds: bool = True,
     ) -> Tuple[List[Any], List[int]]:
-        out_len = len(signature_tree.types)
-        if result is None:
-            final_result = []
-        else:
-            if out_len == 1:
-                final_result = [result]
-            else:
-                result_type = type(result)
-                if result_type is not list and result_type is not tuple:
-                    raise SignatureBodyMismatchError(
-                        "Expected signal to return a list or tuple of arguments"
-                    )
-                final_result = result
-
-        if out_len != len(final_result):
-            raise SignatureBodyMismatchError(
-                f"Signature and function return mismatch, expected {len(signature_tree.types)} arguments but got {len(result)}"
-            )
-
-        if not replace_fds:
-            return final_result, []
-        return replace_fds_with_idx(signature_tree, final_result)
+        return _real_fn_result_to_body(result, signature_tree, replace_fds)
 
     @staticmethod
     def _c_fn_result_to_body(
@@ -559,28 +567,7 @@ def _c_fn_result_to_body(
         wrapped in a list to be a message body. Also they may return fds
         directly for type 'h' which need to be put into an external list."""
         # https://github.com/cython/cython/issues/3327
-        out_len = len(signature_tree.types)
-        if result is None:
-            final_result = []
-        else:
-            if out_len == 1:
-                final_result = [result]
-            else:
-                result_type = type(result)
-                if result_type is not list and result_type is not tuple:
-                    raise SignatureBodyMismatchError(
-                        "Expected signal to return a list or tuple of arguments"
-                    )
-                final_result = result
-
-        if out_len != len(final_result):
-            raise SignatureBodyMismatchError(
-                f"Signature and function return mismatch, expected {len(signature_tree.types)} arguments but got {len(result)}"
-            )
-
-        if not replace_fds:
-            return final_result, []
-        return replace_fds_with_idx(signature_tree, final_result)
+        return _real_fn_result_to_body(result, signature_tree, replace_fds)
 
     @staticmethod
     def _handle_signal(