diff --git a/.ci/isabelle4isadof/Dockerfile b/.ci/isabelle4isadof/Dockerfile
index 1bf3583..d9e6920 100755
--- a/.ci/isabelle4isadof/Dockerfile
+++ b/.ci/isabelle4isadof/Dockerfile
@@ -24,7 +24,7 @@
 #
 # SPDX-License-Identifier: BSD-2-Clause
 
-FROM logicalhacking/lh-docker-isabelle:isabelle2020
+FROM logicalhacking/lh-docker-isabelle:isabelle2021
 
 WORKDIR /home/isabelle
 COPY thy_output.ML /home/isabelle/Isabelle/src/Pure/Thy