diff --git a/rowers/interactiveplots.py b/rowers/interactiveplots.py
index 5474686c..7bc73bf4 100644
--- a/rowers/interactiveplots.py
+++ b/rowers/interactiveplots.py
@@ -619,7 +619,7 @@ def interactive_otwcpchart(powerdf,promember=0):
     # there is no Paul's law for OTW
 
     # Fit the data to thee parameter CP model
-    fitfunc = lambda pars,x: pars[0]/(1+(x/pars[2])) + pars[1]/(1+(x/abs(pars[3])))
+    fitfunc = lambda pars,x: abs(pars[0])/(1+(x/abs(pars[2]))) + abs(pars[1])/(1+(x/abs(pars[3])))
     errfunc = lambda pars,x,y: fitfunc(pars,x)-y
     
     p0 = [500,350,10,8000]