diff --git a/bob/bio/base/transformers/preprocessor.py b/bob/bio/base/transformers/preprocessor.py
index acb7824d888643c5acbb084619307bceb10ba494..fcb5dfab37b8ac2f2e687d7e67286c535aa017ed 100644
--- a/bob/bio/base/transformers/preprocessor.py
+++ b/bob/bio/base/transformers/preprocessor.py
@@ -11,7 +11,7 @@ class PreprocessorTransformer(TransformerMixin, BaseEstimator):
     Parameters
     ----------
 
-      instance: ``collections.callable``
+      instance: object
          Instance of `bob.bio.base.preprocessor.Preprocessor`