diff --git a/rowers/models.py b/rowers/models.py
index 1ff7545f..c70e9c66 100644
--- a/rowers/models.py
+++ b/rowers/models.py
@@ -4249,7 +4249,10 @@ class RowerPowerForm(ModelForm):
         self.initial['otwftp'] = int((1-0.01*self.instance.otwslack)*self.instance.ftp)
 
     def save(self, *args, **kwargs):
-        otwslack = -100.*(self.cleaned_data['otwftp']-self.cleaned_data['ftp'])/(self.cleaned_data['ftp'])
+        try:
+            otwslack = -100.*(self.cleaned_data['otwftp']-self.cleaned_data['ftp'])/(self.cleaned_data['ftp'])
+        except:
+            otwslack = 10.
         self.instance.otwslack = otwslack
         return super(RowerPowerForm, self).save(*args, **kwargs)