diff --git a/packaging/hydra.nix b/packaging/hydra.nix
index 20605418b..3a31314f7 100644
--- a/packaging/hydra.nix
+++ b/packaging/hydra.nix
@@ -63,7 +63,6 @@ let
         "nix-cli"
         "nix-functional-tests"
         "nix-json-schema-checks"
-        "nix-kaitai-struct-checks"
       ]
       ++ lib.optionals enableBindings [
         "nix-perl-bindings"
@@ -73,6 +72,7 @@ let
         "nix-manual-manpages-only"
         "nix-internal-api-docs"
         "nix-external-api-docs"
+        "nix-kaitai-struct-checks"
       ]
     );
 in
diff --git a/src/libexpr-tests/nix_api_expr.cc b/src/libexpr-tests/nix_api_expr.cc
index fdd09a449..c7e246c72 100644
--- a/src/libexpr-tests/nix_api_expr.cc
+++ b/src/libexpr-tests/nix_api_expr.cc
@@ -232,22 +232,22 @@ TEST_F(nix_api_expr_test, nix_expr_realise_context)
     nix_realised_string_free(r);
 }
 
-const char * SAMPLE_USER_DATA = "whatever";
+static const char SAMPLE_USER_DATA = 0;
 
 static void
 primop_square(void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
 {
     assert(context);
     assert(state);
-    assert(user_data == SAMPLE_USER_DATA);
+    assert(user_data == &SAMPLE_USER_DATA);
     auto i = nix_get_int(context, args[0]);
     nix_init_int(context, ret, i * i);
 }
 
 TEST_F(nix_api_expr_test, nix_expr_primop)
 {
-    PrimOp * primop =
-        nix_alloc_primop(ctx, primop_square, 1, "square", nullptr, "square an integer", (void *) SAMPLE_USER_DATA);
+    PrimOp * primop = nix_alloc_primop(
+        ctx, primop_square, 1, "square", nullptr, "square an integer", const_cast<char *>(&SAMPLE_USER_DATA));
     assert_ctx_ok();
     nix_value * primopValue = nix_alloc_value(ctx, state);
     assert_ctx_ok();
@@ -273,7 +273,7 @@ primop_repeat(void * user_data, nix_c_context * context, EvalState * state, nix_
 {
     assert(context);
     assert(state);
-    assert(user_data == SAMPLE_USER_DATA);
+    assert(user_data == &SAMPLE_USER_DATA);
 
     // Get the string to repeat
     std::string s;
@@ -295,8 +295,8 @@ primop_repeat(void * user_data, nix_c_context * context, EvalState * state, nix_
 
 TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_multiple_calls)
 {
-    PrimOp * primop =
-        nix_alloc_primop(ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", (void *) SAMPLE_USER_DATA);
+    PrimOp * primop = nix_alloc_primop(
+        ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", const_cast<char *>(&SAMPLE_USER_DATA));
     assert_ctx_ok();
     nix_value * primopValue = nix_alloc_value(ctx, state);
     assert_ctx_ok();
@@ -330,8 +330,8 @@ TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_multiple_calls)
 
 TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_single_call)
 {
-    PrimOp * primop =
-        nix_alloc_primop(ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", (void *) SAMPLE_USER_DATA);
+    PrimOp * primop = nix_alloc_primop(
+        ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", const_cast<char *>(&SAMPLE_USER_DATA));
     assert_ctx_ok();
     nix_value * primopValue = nix_alloc_value(ctx, state);
     assert_ctx_ok();
diff --git a/tests/nixos/fetchers-substitute.nix b/tests/nixos/fetchers-substitute.nix
index dc3bb7c02..5363f8e72 100644
--- a/tests/nixos/fetchers-substitute.nix
+++ b/tests/nixos/fetchers-substitute.nix
@@ -47,6 +47,7 @@
     { nodes }: # python
     ''
       import json
+      import os
 
       start_all()
 
@@ -118,9 +119,8 @@
 
       # Get the NAR hash of the unpacked tarball in SRI format
       path_info_json = substituter.succeed(f"nix path-info --json-format 2 --json {tarball_store_path}").strip()
-      path_info_dict = json.loads(path_info_json)
-      # nix path-info returns a dict with store paths as keys
-      narHash_obj = path_info_dict[tarball_store_path]["narHash"]
+      path_info_dict = json.loads(path_info_json)["info"]
+      narHash_obj = path_info_dict[os.path.basename(tarball_store_path)]["narHash"]
       # Convert from structured format {"algorithm": "sha256", "format": "base16", "hash": "..."} to SRI string
       tarball_hash_sri = substituter.succeed(f"nix hash convert --to sri {narHash_obj['algorithm']}:{narHash_obj['hash']}").strip()
       print(f"Tarball NAR hash (SRI): {tarball_hash_sri}")