diff --git a/plan.go b/plan.go
index de2b67c..bcf44fa 100644
--- a/plan.go
+++ b/plan.go
@@ -182,7 +182,10 @@ func toCtyValue(a any) (cty.Value, error) {
 			}
 			sv = append(sv, v)
 		}
-		return cty.ListVal(sv), nil
+
+		// Always use a tuple over a list. Tuples are heterogeneous typed lists, which is
+		// more robust. Functionally equivalent for our use case of looking up values.
+		return cty.TupleVal(sv), nil
 	case reflect.Map:
 		if av.Type().Key().Kind() != reflect.String {
 			return cty.NilVal, fmt.Errorf("map keys must be string, found %q", av.Type().Key().Kind())
diff --git a/plan_internal_test.go b/plan_internal_test.go
new file mode 100644
index 0000000..fe86220
--- /dev/null
+++ b/plan_internal_test.go
@@ -0,0 +1,32 @@
+package preview
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/zclconf/go-cty/cty"
+)
+
+func Test_toCtyValue(t *testing.T) {
+	t.Parallel()
+
+	t.Run("EmptyList", func(t *testing.T) {
+		t.Parallel()
+		val, err := toCtyValue([]any{})
+		require.NoError(t, err)
+		require.True(t, val.Type().IsTupleType())
+	})
+
+	t.Run("HeterogeneousList", func(t *testing.T) {
+		t.Parallel()
+		val, err := toCtyValue([]any{5, "hello", true})
+		require.NoError(t, err)
+		require.True(t, val.Type().IsTupleType())
+		require.Equal(t, 3, val.LengthInt())
+		require.True(t, val.Equals(cty.TupleVal([]cty.Value{
+			cty.NumberIntVal(5),
+			cty.StringVal("hello"),
+			cty.BoolVal(true),
+		})).True())
+	})
+}