diff --git a/‎gitlab/_backends/requests_backend.py‎
Lines changed: 8 additions & 4 deletions b/‎gitlab/_backends/requests_backend.py‎
Lines changed: 8 additions & 4 deletions
diff --git a/‎gitlab/v4/objects/groups.py‎
Lines changed: 2 additions & 2 deletions b/‎gitlab/v4/objects/groups.py‎
Lines changed: 2 additions & 2 deletions
diff --git a/‎tests/unit/_backends/test_requests_backend.py‎
Lines changed: 17 additions & 4 deletions b/‎tests/unit/_backends/test_requests_backend.py‎
Lines changed: 17 additions & 4 deletions
@@ -70,17 +70,21 @@ def prepare_send_data(
             if post_data is None:
                 post_data = {}
             else:
-                # booleans does not exists for data (neither for MultipartEncoder):
-                # cast to string int to avoid: 'bool' object has no attribute 'encode'
+                # When creating a `MultipartEncoder` instance with data-types
+                # which don't have an `encode` method it will cause an error:
+                #       object has no attribute 'encode'
+                # So convert common non-string types into strings.
                 if TYPE_CHECKING:
                     assert isinstance(post_data, dict)
                 for k, v in post_data.items():
                     if isinstance(v, bool):
-                        post_data[k] = str(int(v))
+                        v = int(v)
+                    if isinstance(v, (complex, float, int)):
+                        post_data[k] = str(v)
             post_data["file"] = files.get("file")
             post_data["avatar"] = files.get("avatar")
 
-            data = MultipartEncoder(post_data)
+            data = MultipartEncoder(fields=post_data)
             return SendData(data=data, content_type=data.content_type)
 
         if raw and post_data:
 
@@ -378,7 +378,7 @@ def import_group(
         file: BinaryIO,
         path: str,
         name: str,
-        parent_id: Optional[str] = None,
+        parent_id: Optional[Union[int, str]] = None,
         **kwargs: Any,
     ) -> Union[Dict[str, Any], requests.Response]:
         """Import a group from an archive file.
@@ -399,7 +399,7 @@ def import_group(
             A representation of the import status.
         """
         files = {"file": ("file.tar.gz", file, "application/octet-stream")}
-        data = {"path": path, "name": name}
+        data: Dict[str, Any] = {"path": path, "name": name}
         if parent_id is not None:
             data["parent_id"] = parent_id
 
 
@@ -25,14 +25,27 @@ def test_senddata_json_and_data(self) -> None:
 
 
 class TestRequestsBackend:
-    def test_prepare_send_data_str_parentid(self) -> None:
-        file = "12345"
-        files = {"file": ("file.tar.gz", file, "application/octet-stream")}
-        post_data = {"parent_id": "12"}
+    @pytest.mark.parametrize(
+        "test_data,expected",
+        [
+            (False, "0"),
+            (True, "1"),
+            ("12", "12"),
+            (12, "12"),
+            (12.0, "12.0"),
+            (complex(-2, 7), "(-2+7j)"),
+        ],
+    )
+    def test_prepare_send_data_non_strings(self, test_data, expected) -> None:
+        assert isinstance(expected, str)
+        files = {"file": ("file.tar.gz", "12345", "application/octet-stream")}
+        post_data = {"test_data": test_data}
 
         result = requests_backend.RequestsBackend.prepare_send_data(
             files=files, post_data=post_data, raw=False
         )
         assert result.json is None
         assert result.content_type.startswith("multipart/form-data")
         assert isinstance(result.data, MultipartEncoder)
+        assert isinstance(result.data.fields["test_data"], str)
+        assert result.data.fields["test_data"] == expected