diff --git a/tex/latex/biblatex/bbx/authoryear.bbx b/tex/latex/biblatex/bbx/authoryear.bbx
index d8e156cd3..f1b8f34c1 100644
--- a/tex/latex/biblatex/bbx/authoryear.bbx
+++ b/tex/latex/biblatex/bbx/authoryear.bbx
@@ -9,7 +9,7 @@
      \renewbibmacro*{bbx:savehash}{\savefield{fullhash}{\bbx@lasthash}}}
     {\renewbibmacro*{bbx:savehash}{}}}
 
-\DeclareBiblatexOption{global,type,entry}[boolean]{mergedate}[true]{%
+\DeclareBiblatexOption{global,type,entry}[string]{mergedate}[true]{%
   \ifcsdef{bbx@opt@mergedate@#1}
     {\csuse{bbx@opt@mergedate@#1}}
     {\PackageError{biblatex}