diff --git a/line.py b/line.py
index e12e19f..410d5f2 100644
--- a/line.py
+++ b/line.py
@@ -91,7 +91,8 @@ class Line(Workflow, ModelSQL, ModelView):
                 STATES['readonly'],
                 Eval('bookingtype', '').in_(['spin', 'spout']),
                 ),
-        }, depends=DEPENDS+['currency_digits', 'bookingtype'])
+        }, depends=DEPENDS+['currency_digits', 'bookingtype'],
+        domain=[('amount', '>=', Decimal('0.0'))])
     debit = fields.Numeric(string='Debit', digits=(16, Eval('currency_digits', 2)),
         required=True, readonly=True, depends=['currency_digits'])
     credit = fields.Numeric(string='Credit', digits=(16, Eval('currency_digits', 2)),
@@ -183,6 +184,9 @@ class Line(Workflow, ModelSQL, ModelView):
             ('state_val',
                 Check(t, t.state.in_(['edit', 'check', 'done'])),
                 'cashbook.msg_line_wrong_state_value'),
+            ('amount_val',
+                Check(t, t.amount >= Decimal('0.0')),
+                'cashbook.msg_line_must_positive'),
             ])
         cls._transitions |= set((
                 ('edit', 'check'),
diff --git a/locale/de.po b/locale/de.po
index e24c014..39e5aaf 100644
--- a/locale/de.po
+++ b/locale/de.po
@@ -150,6 +150,10 @@ msgctxt "model:ir.message,text:msg_line_invalid_category"
 msgid "The category of the booking line '%(recname)s' does not match the posting type '%(booktype)s'."
 msgstr "Die Kategorie der Buchungszeile '%(recname)s' paßt nicht zum Buchungstyp '%(booktype)s'."
 
+msgctxt "model:ir.message,text:msg_line_must_positive"
+msgid "The amount must be positive."
+msgstr "Der Betrag muß positiv sein."
+
 
 #############
 # res.group #
diff --git a/message.xml b/message.xml
index 8fea230..18ce400 100644
--- a/message.xml
+++ b/message.xml
@@ -113,6 +113,9 @@ full copyright notices and license terms. -->
         <record model="ir.message" id="msg_line_invalid_category">
             <field name="text">The category of the booking line '%(recname)s' does not match the posting type '%(booktype)s'.</field>
         </record>
+        <record model="ir.message" id="msg_line_must_positive">
+            <field name="text">The amount must be positive.</field>
+        </record>
 
     </data>
 </tryton>