diff --git a/ptbposter/ptbposter.cls b/ptbposter/ptbposter.cls
index 6acf9d8d64fd8e4207fad051bc49b41d956c2c19..3b04724fc853bf4c53e08f604ad5b06e2b40cf79 100644
--- a/ptbposter/ptbposter.cls
+++ b/ptbposter/ptbposter.cls
@@ -102,7 +102,7 @@
   \def\PTB@target@size{a1}
 }
 \DeclareOption{a0paper}{%
-  \OptionNotUsed
+  \let\PTB@target@size=\@empty
 }
 \DeclareOption{landscape}{%
   \PTBlandscapetrue