diff --git a/openlcb/__init__.py b/openlcb/__init__.py
index 0bfae49..db88c0e 100644
--- a/openlcb/__init__.py
+++ b/openlcb/__init__.py
@@ -12,3 +12,8 @@ def only_hex_pairs(value):
     functions (less restrictive).
     """
     return hex_pairs_rc.fullmatch(value)
+
+
+def emit_cast(value):
+    """Show type and value, such as for debug output."""
+    return "{}({})".format(type(value).__name__, repr(value))
diff --git a/openlcb/conventions.py b/openlcb/conventions.py
index 9614d98..cd29dad 100644
--- a/openlcb/conventions.py
+++ b/openlcb/conventions.py
@@ -1,6 +1,9 @@
 from logging import getLogger
 
-from openlcb import only_hex_pairs
+from openlcb import (
+    only_hex_pairs,
+    emit_cast,
+)
 
 logger = getLogger(__name__)
 
@@ -8,6 +11,10 @@
 
 
 def hex_to_dotted_lcc_id(hex_s):
+    if (not isinstance(hex_s, str)) or (len(hex_s) != 12):
+        raise ValueError(
+            "Only 6 hex pairs (12 characters) allowed, but got {}."
+            .format(emit_cast(hex_s)))
     return LCC_ID_SEP.join([hex_s[i*2:i*2+2] for i in range(len(hex_s)//2)])
 
 
diff --git a/tests/test_conventions.py b/tests/test_conventions.py
index ea14ef7..6ccd0e7 100644
--- a/tests/test_conventions.py
+++ b/tests/test_conventions.py
@@ -82,6 +82,14 @@ def test_hex_to_dotted_lcc_id(self):
         self.assertEqual(hex_to_dotted_lcc_id("02015700049C"),
                          "02.01.57.00.04.9C")
 
+    def test_hex_to_dotted_lcc_id_fail(self):
+        exception = None
+        try:
+            _ = hex_to_dotted_lcc_id("2015700049C")
+        except ValueError as ex:
+            exception = ex
+        self.assertIsInstance(exception, ValueError)
+
 
 if __name__ == '__main__':
     unittest.main()