diff --git a/ptbletter/ptbletter.cls b/ptbletter/ptbletter.cls
index 7dc2d3c315c258ed381fa4620f3ef8bddb8b9b83..5377d7bbe43baba0fef52fd6f64536ae1399b968 100644
--- a/ptbletter/ptbletter.cls
+++ b/ptbletter/ptbletter.cls
@@ -122,6 +122,8 @@
 \defineshorthand{"'}{\closeautoquote}
 \AtEndOfClass{\shorthandon{"}}
 
+\RequirePackage{amsmath,siunitx-PTB}
+
 \@setplength{toaddrhpos}{72bp}
 \@setplength{toaddrvpos}{124bp}
 %\@setplength{toaddrwidth}{85mm}