diff --git a/build.sh b/build.sh
index 98de882..a889171 100755
--- a/build.sh
+++ b/build.sh
@@ -37,4 +37,4 @@ LATEST="2019"
 docker build -t logicalhacking/debian4isabelle debian4isabelle
 
 # Generate Isabelle image(s)
-source isabelle/hooks/build
+( cd isabelle && source hooks/build)