diff --git a/src/nested_pandas/nestedframe/core.py b/src/nested_pandas/nestedframe/core.py
index c7f2b38..3f28a5e 100644
--- a/src/nested_pandas/nestedframe/core.py
+++ b/src/nested_pandas/nestedframe/core.py
@@ -210,6 +210,10 @@ def from_flat(cls, df, base_columns, nested_columns=None, index=None, name="nest
         # drop duplicates on index
         out_df = df[base_columns][~df.index.duplicated(keep="first")]
 
+        # Convert df to NestedFrame if needed
+        if not isinstance(out_df, NestedFrame):
+            out_df = NestedFrame(out_df)
+
         # add nested
         if nested_columns is None:
             nested_columns = [col for col in df.columns if col not in base_columns]
diff --git a/tests/nested_pandas/nestedframe/test_nestedframe.py b/tests/nested_pandas/nestedframe/test_nestedframe.py
index 097ce1b..95421b3 100644
--- a/tests/nested_pandas/nestedframe/test_nestedframe.py
+++ b/tests/nested_pandas/nestedframe/test_nestedframe.py
@@ -285,13 +285,21 @@ def test_add_nested_for_empty_df():
     assert_frame_equal(new_base.nested.nest.to_flat(), nested.astype(pd.ArrowDtype(pa.float64())))
 
 
+@pytest.mark.parametrize("pandas", [False, True])
 @pytest.mark.parametrize("index", [None, "a", "c"])
-def test_from_flat(index):
+def test_from_flat(index, pandas):
     """Test the NestedFrame.from_flat functionality"""
-    nf = NestedFrame(
-        {"a": [1, 1, 1, 2, 2], "b": [2, 2, 2, 4, 4], "c": [1, 2, 3, 4, 5], "d": [2, 4, 6, 8, 10]},
-        index=[0, 0, 0, 1, 1],
-    )
+
+    if pandas:
+        nf = pd.DataFrame(
+            {"a": [1, 1, 1, 2, 2], "b": [2, 2, 2, 4, 4], "c": [1, 2, 3, 4, 5], "d": [2, 4, 6, 8, 10]},
+            index=[0, 0, 0, 1, 1],
+        )
+    else:
+        nf = NestedFrame(
+            {"a": [1, 1, 1, 2, 2], "b": [2, 2, 2, 4, 4], "c": [1, 2, 3, 4, 5], "d": [2, 4, 6, 8, 10]},
+            index=[0, 0, 0, 1, 1],
+        )
 
     out_nf = NestedFrame.from_flat(nf, base_columns=["a", "b"], index=index, name="new_nested")