diff --git a/src/sbmlsim/fit/analysis.py b/src/sbmlsim/fit/analysis.py
index c8a6993..2981b2d 100644
--- a/src/sbmlsim/fit/analysis.py
+++ b/src/sbmlsim/fit/analysis.py
@@ -534,7 +534,7 @@ def _datapoints_df(self, x: np.ndarray) -> pd.DataFrame:
             residuals = res_data["residuals"][k]
             for ix in range(len(y_obs)):
                 if not y_ref_err_type:
-                    y_err = np.NaN
+                    y_err = np.nan
                 else:
                     y_err = y_ref_err[ix]
 
diff --git a/src/sbmlsim/fit/optimization.py b/src/sbmlsim/fit/optimization.py
index 34908fc..9ae8a04 100644
--- a/src/sbmlsim/fit/optimization.py
+++ b/src/sbmlsim/fit/optimization.py
@@ -372,7 +372,7 @@ def initialize(
                 # handle missing data (0.0 and NaN)
                 if y_ref_err is not None:
                     # remove 0.0 from y-error
-                    y_ref_err[(y_ref_err == 0.0)] = np.NAN
+                    y_ref_err[(y_ref_err == 0.0)] = np.nan
                     if np.all(np.isnan(y_ref_err)):
                         # handle special case of all NaN errors
                         logger.warning(