diff --git a/nix/packages/backend.nix b/nix/packages/backend.nix
index c662526..e00b91b 100644
--- a/nix/packages/backend.nix
+++ b/nix/packages/backend.nix
@@ -1,6 +1,13 @@
 { php84, lib }:
 let
-  php = php84;
+  # Laravel connects to Postgres via PDO, so the runtime PHP must
+  # ship pdo_pgsql. The default php84 set ships pdo, pdo_mysql, and
+  # pdo_sqlite but not pdo_pgsql. buildEnv gives us a php with the
+  # extra extension that flows through to passthru.php and the
+  # composer build hooks.
+  php = php84.buildEnv {
+    extensions = ({ enabled, all }: enabled ++ [ all.pdo_pgsql ]);
+  };
 in
 php.buildComposerProject (finalAttrs: {
   pname = "tide-backend";