diff --git a/pytools/__init__.py b/pytools/__init__.py
index 0ffd8f4fe50eb89df6771acdd1f483820653f4de..bd654ea5e68ae613782bcbe2fcd5672b5a9e8895 100644
--- a/pytools/__init__.py
+++ b/pytools/__init__.py
@@ -2176,9 +2176,9 @@ class UniqueNameGenerator:
 
     def add_name(self, name: str) -> None:
         if self.is_name_conflicting(name):
-            raise ValueError("name '%s' conflicts with existing names")
+            raise ValueError("name '%s' conflicts with existing names" % name)
         if not name.startswith(self.forced_prefix):
-            raise ValueError("name '%s' does not start with required prefix")
+            raise ValueError("name '%s' does not start with required prefix" % name)
 
         self.existing_names.add(name)
         self._name_added(name)