diff --git a/lob_python/model/letter.py b/lob_python/model/letter.py
index 426c47c..984dbca 100755
--- a/lob_python/model/letter.py
+++ b/lob_python/model/letter.py
@@ -97,8 +97,15 @@ class Letter(ModelNormal):
     }
 
     validations = {
-        ('tracking_events',): {
-            'max_items': 0,
+        ('template_id',): {
+            'regex': {
+                'pattern': r'^tmpl_[a-zA-Z0-9]+$',  # noqa: E501
+            },
+        },
+        ('template_version_id',): {
+            'regex': {
+                'pattern': r'^vrsn_[a-zA-Z0-9]+$',  # noqa: E501
+            },
         },
     }
 
diff --git a/setup.py b/setup.py
index 6dca4ff..3dfbd2e 100644
--- a/setup.py
+++ b/setup.py
@@ -11,7 +11,7 @@
 from setuptools import setup, find_packages  # noqa: H301
 
 NAME = "lob-python"
-VERSION = "5.1.2"
+VERSION = "5.1.3"
 # To install the library, run the following
 #
 # python setup.py install