diff --git a/guix-data-service/model/build.scm b/guix-data-service/model/build.scm
index 3d38e67..0959af8 100644
--- a/guix-data-service/model/build.scm
+++ b/guix-data-service/model/build.scm
@@ -53,7 +53,7 @@
             '())
       ,@(if system
             `(("package_derivations.system_id = $" .
-               ,(system->system-id conn system)))
+               ,(number->string (system->system-id conn system))))
             '())
       ,@(if target
             `(("package_derivations.target = $" . ,target))
@@ -146,7 +146,7 @@ ORDER BY status"))
             '())
       ,@(if system
             `(("package_derivations.system_id = $" .
-               ,(system->system-id conn system)))
+               ,(number->string (system->system-id conn system))))
             '())
       ,@(if target
             `(("package_derivations.target = $" . ,target))