diff --git a/bdpy/fig/makeplots2.py b/bdpy/fig/makeplots2.py
index 04915da..d2c8883 100644
--- a/bdpy/fig/makeplots2.py
+++ b/bdpy/fig/makeplots2.py
@@ -593,9 +593,10 @@ def __weird_form_to_long(df, target_col, identify_cols=[]):
     identify_cols: list of str
         A list of columns to keep in the decomposed dataframe, other than target_col.
     """
-    # replace NaN to [np.nan]
-    nans = df[target_col].isna()
-    df[target_col][nans] = [np.nan]
+    # # replace NaN to [np.nan]
+    nan_index = np.where(df[target_col].isna())[0]
+    for i in nan_index:
+        df.at[i, target_col] = [np.nan]
     # Weird form
     df_result = pd.DataFrame()
     for i, row in df.iterrows():