diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp
new file mode 100644
index 000000000..d81cc0710
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix
new file mode 100644
index 000000000..5c6702120
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals-deferred.nix
@@ -0,0 +1,3 @@
+# Tests formals which have internal default values that must be deferred.
+
+({ optional ? defaultValue, defaultValue }: optional) { defaultValue = 42; }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp
new file mode 100644
index 000000000..d81cc0710
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.exp
@@ -0,0 +1 @@
+42
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix
new file mode 100644
index 000000000..c6dd5e9d5
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-internal-formals.nix
@@ -0,0 +1,3 @@
+# Tests formals which have internal default values.
+
+({ defaultValue, optional ? defaultValue }: optional) { defaultValue = 42; }