diff --git a/pvanalytics/util/_fit.py b/pvanalytics/util/_fit.py
index 65a61559..eb33f2a2 100644
--- a/pvanalytics/util/_fit.py
+++ b/pvanalytics/util/_fit.py
@@ -127,4 +127,8 @@ def _quartic(x, a, b, c, e):
     )
     model = _quartic(x, params[0], params[1], params[2], params[3])
     residuals = y - model
-    return 1 - (np.sum(residuals**2) / np.sum((y - np.mean(y))**2))
+    ss_res = np.sum(residuals**2)
+    ss_tot = np.sum((y - np.mean(y))**2)
+    if ss_tot == 0:
+        return 0.0  
+    return 1 - (ss_res / ss_tot)
diff --git a/pvanalytics/util/_functions.py b/pvanalytics/util/_functions.py
index 455dde35..90e2cebe 100644
--- a/pvanalytics/util/_functions.py
+++ b/pvanalytics/util/_functions.py
@@ -14,5 +14,11 @@ def freq_to_timedelta(freq):
     -------
     Timedelta
         Timedelta corresponding to a single period at `freq`.
+        For offsets that cannot be directly converted (e.g. 'D', 'W'),
+        falls back to using the offset's nanosecond representation.
     """
-    return pd.to_timedelta(frequencies.to_offset(freq))
+    offset = frequencies.to_offset(freq)
+    try:
+        return pd.to_timedelta(offset)
+    except ValueError:
+        return pd.Timedelta(offset.nanos, unit='ns')