diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 6945e4dae..50997e096 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -191,7 +191,7 @@ std::ostream & operator << (std::ostream & str, const Pos & pos)
     if (!pos)
         str << "undefined position";
     else
-        str << (format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%") % pos.file % pos.line % pos.column).str();
+        str << (format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%") % (string) pos.file % pos.line % pos.column).str();
     return str;
 }