diff --git a/frappy_psi/picontrol.py b/frappy_psi/picontrol.py
index 73e84c16..ee50bf4c 100644
--- a/frappy_psi/picontrol.py
+++ b/frappy_psi/picontrol.py
@@ -160,7 +160,7 @@ class PImixin(HasOutputModule, Writable):
                     output = omax
             if overflow:
                 # fit a straight line
-                (slope, beg), cov = np.polyfit(range(self._overflow), self._overflow, 1, cov=True)
+                (slope, beg), cov = np.polyfit(range(len(self._overflow)), self._overflow, 1, cov=True)
                 sign = np.copysign(1, overflow)
                 end = beg + slope * len(self._overflow)
                 # reduce the absolute value of overflow by the minimum distance of the fitted