diff --git a/cmem_plugin_ulid/transform/__init__.py b/cmem_plugin_ulid/transform/__init__.py
index 755dfa4..f56b89f 100644
--- a/cmem_plugin_ulid/transform/__init__.py
+++ b/cmem_plugin_ulid/transform/__init__.py
@@ -41,6 +41,8 @@ def __init__(self, number_of_values=1):
         self.number_of_values = number_of_values
 
     def transform(self, inputs: Sequence[Sequence[str]]) -> Sequence[str]:
+        if inputs:
+            raise ValueError("Plugin does not support processing input entities.")
         result = []
         for _ in range(self.number_of_values):
             result += [f"{ULID()}"]