diff --git a/tvix/castore/default.nix b/tvix/castore/default.nix
index 09f6c0bea..efa4da59c 100644
--- a/tvix/castore/default.nix
+++ b/tvix/castore/default.nix
@@ -2,4 +2,7 @@
 
 depot.tvix.crates.workspaceMembers.tvix-castore.build.override {
   runTests = true;
+  testPreRun = ''
+    export SSL_CERT_FILE=${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt;
+  '';
 }