diff --git a/pvanalytics/util/_fit.py b/pvanalytics/util/_fit.py
index 65a61559..479be44a 100644
--- a/pvanalytics/util/_fit.py
+++ b/pvanalytics/util/_fit.py
@@ -127,4 +127,7 @@ 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_tot = np.sum((y - np.mean(y))**2)
+    if ss_tot == 0:
+        return 0.0
+    return 1 - (np.sum(residuals**2) / ss_tot)