diff --git a/qiskit_experiments/curve_analysis/curve_fit.py b/qiskit_experiments/curve_analysis/curve_fit.py
index 7021b07311..26f99476f3 100644
--- a/qiskit_experiments/curve_analysis/curve_fit.py
+++ b/qiskit_experiments/curve_analysis/curve_fit.py
@@ -109,13 +109,14 @@ def fit_func(x, *params):
         )
 
     # Format non-number sigma values
-    if np.all(np.isnan(sigma)):
-        sigma = None
-    else:
-        sigma = np.nan_to_num(sigma)
-        if np.count_nonzero(sigma) != len(sigma):
-            # Sigma = 0 causes zero division error
+    if sigma is not None:
+        if np.all(np.isnan(sigma)):
             sigma = None
+        else:
+            sigma = np.nan_to_num(sigma)
+            if np.count_nonzero(sigma) != len(sigma):
+                # Sigma = 0 causes zero division error
+                sigma = None
 
     # Override scipy.curve_fit default for absolute_sigma=True
     # if sigma is specified.