diff --git a/symposion/reviews/models.py b/symposion/reviews/models.py
index f6234d515a85edaf5230edd32ba39f5bb62f1d6d..c48acf12231c653c4368cab747e94c30e675e43a 100644
--- a/symposion/reviews/models.py
+++ b/symposion/reviews/models.py
@@ -3,8 +3,11 @@ from __future__ import unicode_literals
 from datetime import datetime
 from decimal import Decimal
 
+from django.core.exceptions import ValidationError
+
 from django.db import models
 from django.db.models import Q, F
+from django.db.models import Case, When, Value
 from django.db.models.signals import post_save
 
 from django.contrib.auth.models import User
@@ -16,13 +19,18 @@ from symposion.schedule.models import Presentation
 
 
 def score_expression():
-    return (
+    score = (
         (2 * F("plus_two") + F("plus_one")) -
         (F("minus_one") + 2 * F("minus_two"))
     ) / (
         F("vote_count") - F("abstain") * 1.0
     )
 
+    return Case(
+        When(vote_count=F("abstain"), then=Value("0")),  # no divide by zero
+        default=score,
+    )
+
 
 class Votes(object):
     ABSTAIN = "0"
@@ -120,10 +128,21 @@ class Review(models.Model):
     # No way to encode "-0" vs. "+0" into an IntegerField, and I don't feel
     # like some complicated encoding system.
     vote = models.CharField(max_length=2, blank=True, choices=VOTES.CHOICES, verbose_name=_("Vote"))
-    comment = models.TextField(verbose_name=_("Comment"))
+    comment = models.TextField(
+        blank=True,
+        verbose_name=_("Comment")
+    )
     comment_html = models.TextField(blank=True)
     submitted_at = models.DateTimeField(default=datetime.now, editable=False, verbose_name=_("Submitted at"))
 
+    def clean(self):
+        err = {}
+        if self.vote != VOTES.ABSTAIN and not self.comment.strip():
+            err["comment"] = ValidationError(_("You must provide a comment"))
+
+        if err:
+            raise ValidationError(err)
+
     def save(self, **kwargs):
         self.comment_html = parse(self.comment)
         if self.vote: