diff --git a/inema/wpint.py b/inema/wpint.py
index cd8788b..37faa4b 100644
--- a/inema/wpint.py
+++ b/inema/wpint.py
@@ -222,6 +222,8 @@ class WarenpostInt(object):
             raise ValueError('Maximum item gross weight is 2000g')
         if len(currency) != 3:
             raise ValueError('Currency must be expressed as 3-digit ISO-4217 code')
+        if contents and len(contents) > 5:
+            raise ValueError('Custom Contents must not contain more than 5 lines')
         ret = {
             'product': str(product),
             'serviceLevel': 'STANDARD',