diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index a1d8b751..37d7dcbc 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -8,10 +8,16 @@ This document describes changes between each past release.
 
 Added
 =====
+
 - Add CORS headers in the API (#407)
 - Document database migrations (#390)
 - Allow basic math operations in amount field (#413)
 
+Fixed
+=====
+
+- Do not allow negative weights on users (#366)
+
 3.0 (2018-11-25)
 ----------------
 
diff --git a/ihatemoney/forms.py b/ihatemoney/forms.py
index 5374fd9f..be04c8f8 100644
--- a/ihatemoney/forms.py
+++ b/ihatemoney/forms.py
@@ -2,7 +2,7 @@ from flask_wtf.form import FlaskForm
 from wtforms.fields.core import SelectField, SelectMultipleField
 from wtforms.fields.html5 import DateField, DecimalField
 from wtforms.fields.simple import PasswordField, SubmitField, TextAreaField, StringField
-from wtforms.validators import Email, Required, ValidationError, EqualTo
+from wtforms.validators import Email, Required, ValidationError, EqualTo, NumberRange
 from flask_babel import lazy_gettext as _
 from flask import request
 from werkzeug.security import generate_password_hash
@@ -174,9 +174,11 @@ class BillForm(FlaskForm):
 
 
 class MemberForm(FlaskForm):
-
     name = StringField(_("Name"), validators=[Required()])
-    weight = CommaDecimalField(_("Weight"), default=1)
+
+    weight_validators = [NumberRange(min=0.1, message=_("Weights should be positive"))]
+    weight = CommaDecimalField(_("Weight"), default=1,
+                               validators=weight_validators)
     submit = SubmitField(_("Add"))
 
     def __init__(self, project, edit=False, *args, **kwargs):
diff --git a/ihatemoney/messages.pot b/ihatemoney/messages.pot
index ba81d225..fb69539e 100644
--- a/ihatemoney/messages.pot
+++ b/ihatemoney/messages.pot
@@ -79,6 +79,9 @@ msgstr ""
 msgid "Weight"
 msgstr ""
 
+msgid "Weights should be positive"
+msgstr ""
+
 msgid "Add"
 msgstr ""
 
diff --git a/ihatemoney/migrations/versions/a67119aa3ee5_migrate_negative_weights.py b/ihatemoney/migrations/versions/a67119aa3ee5_migrate_negative_weights.py
new file mode 100644
index 00000000..ec234706
--- /dev/null
+++ b/ihatemoney/migrations/versions/a67119aa3ee5_migrate_negative_weights.py
@@ -0,0 +1,38 @@
+"""Migrate negative weights
+
+Revision ID: a67119aa3ee5
+Revises: afbf27e6ef20
+Create Date: 2018-12-25 18:34:20.220844
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'a67119aa3ee5'
+down_revision = 'afbf27e6ef20'
+
+from alembic import op
+import sqlalchemy as sa
+# Snapshot of the person table
+person_helper = sa.Table(
+    'person', sa.MetaData(),
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('project_id', sa.String(length=64), nullable=True),
+    sa.Column('name', sa.UnicodeText(), nullable=True),
+    sa.Column('activated', sa.Boolean(), nullable=True),
+    sa.Column('weight', sa.Float(), nullable=True),
+    sa.ForeignKeyConstraint(['project_id'], ['project.id'], ),
+    sa.PrimaryKeyConstraint('id')
+)
+
+
+def upgrade():
+    op.execute(
+        person_helper.update()
+        .where(person_helper.c.weight <= 0)
+        .values(weight=1)
+    )
+
+
+def downgrade():
+    # Downgrade path is not possible, because information has been lost.
+    pass
diff --git a/ihatemoney/models.py b/ihatemoney/models.py
index 3c36e76c..b9cff4f8 100644
--- a/ihatemoney/models.py
+++ b/ihatemoney/models.py
@@ -376,8 +376,9 @@ class Bill(db.Model):
     def pay_each(self):
         """Compute what each share has to pay"""
         if self.owers:
-            # FIXME: SQL might dot that more efficiently
-            return self.amount / sum(i.weight for i in self.owers)
+            # FIXME: SQL might do that more efficiently
+            weights = sum(i.weight for i in self.owers)
+            return self.amount / weights
         else:
             return 0
 
diff --git a/ihatemoney/tests/tests.py b/ihatemoney/tests/tests.py
index 9f9d8fab..d29ec628 100644
--- a/ihatemoney/tests/tests.py
+++ b/ihatemoney/tests/tests.py
@@ -621,6 +621,18 @@ class BudgetTestCase(IhatemoneyTestCase):
         resp = self.client.get("/raclette/")
         self.assertNotIn('extra-info', resp.data.decode('utf-8'))
 
+    def test_negative_weight(self):
+        self.post_project("raclette")
+
+        # Add one user and edit it to have a negative share
+        self.client.post("/raclette/members/add", data={'name': 'alexis'})
+        resp = self.client.post("/raclette/members/1/edit", data={'name': 'alexis', 'weight': -1})
+
+        # An error should be generated, and its weight should still be 1.
+        self.assertIn('<p class="alert alert-danger">', resp.data.decode('utf-8'))
+        self.assertEqual(len(models.Project.query.get('raclette').members), 1)
+        self.assertEqual(models.Project.query.get('raclette').members[0].weight, 1)
+
     def test_rounding(self):
         self.post_project("raclette")
 
diff --git a/ihatemoney/translations/fr/LC_MESSAGES/messages.po b/ihatemoney/translations/fr/LC_MESSAGES/messages.po
index 2516d568..d610145f 100644
--- a/ihatemoney/translations/fr/LC_MESSAGES/messages.po
+++ b/ihatemoney/translations/fr/LC_MESSAGES/messages.po
@@ -105,6 +105,9 @@ msgstr "Nom"
 msgid "Weight"
 msgstr "Parts"
 
+msgid "Weights should be positive"
+msgstr "Les parts doivent être positives"
+
 msgid "Add"
 msgstr "Ajouter"