diff --git a/pkg/invoices.go b/pkg/invoices.go
index f68e665..a40a9be 100644
--- a/pkg/invoices.go
+++ b/pkg/invoices.go
@@ -81,7 +81,7 @@ func ServeInvoice(w http.ResponseWriter, r *http.Request, params httprouter.Para
 		return
 	}
 	if pdf {
-		cmd := exec.Command("weasyprint", "--stylesheet", "web/static/invoice.css", "-", "-")
+		cmd := exec.Command("weasyprint", "--format", "pdf", "--stylesheet", "web/static/invoice.css", "-", "-")
 		var stderr bytes.Buffer
 		cmd.Stderr = &stderr
 		stdin, err := cmd.StdinPipe()