diff --git a/ptbposter/ptbposter-old-engines.clo b/ptbposter/ptbposter-old-engines.clo
index bcac6c536ad660529b9f115cfdb5b9ee94369adf..f86b9541d7f0d4b4cc7ca7e87c05f0cda0f5c1c5 100644
--- a/ptbposter/ptbposter-old-engines.clo
+++ b/ptbposter/ptbposter-old-engines.clo
@@ -12,8 +12,15 @@
 \ifPTB@serif
   \RequirePackage{qpxmath}
 \else
-  \renewcommand*\familydefault{\sfdefault} 
-  \RequirePackage{sfmath}
+  %%%\renewcommand*\familydefault{\sfdefault} 
+  %%%\RequirePackage{sfmath}
+  \@latex@error{works only with `LuaLaTeX' or `XeLaTeX'}{%
+    type:\MessageBreak 
+    lualatex \jobname.tex\MessageBreak
+    or\MessageBreak 
+    xelatex \jobname.tex
+  }%
+  \@@end 
 \fi
 
 \endinput