diff --git a/examples/technical_report/TR_my_commented_isabelle/document/preamble.tex b/examples/technical_report/TR_my_commented_isabelle/document/preamble.tex
index 0919e95c..1aebc72c 100755
--- a/examples/technical_report/TR_my_commented_isabelle/document/preamble.tex
+++ b/examples/technical_report/TR_my_commented_isabelle/document/preamble.tex
@@ -13,6 +13,7 @@
 
 %% This is a placeholder for user-specific configuration and packages. 
 
+\renewcommand{\isasymtheta}{\texorpdfstring{\isamath{\vartheta}}{ϑ}}
 \usepackage{prooftree}
 \title{<TITLE>}
 \author{<AUTHOR>}