diff --git a/tests/test_xpath.cpp b/tests/test_xpath.cpp
index 5cafb47..fe1e312 100644
--- a/tests/test_xpath.cpp
+++ b/tests/test_xpath.cpp
@@ -449,16 +449,41 @@ TEST(xpath_out_of_memory_evaluate_substring)
 	CHECK_ALLOC_FAIL(CHECK(q.evaluate_string(0, 0, xml_node()) == 1));
 }
 
-/*
-TEST_XML(xpath_out_of_memory_evaluate_union, "<node><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/></node>")
+TEST_XML(xpath_out_of_memory_evaluate_union, "<node />")
 {
-	test_runner::_memory_fail_threshold = 32768 + 4096 * 2;
+	// left hand side: size * sizeof(xpath_node) (8 on 32-bit, 16 on 64-bit)
+	// right hand side: same
+	// to make sure that when we append right hand side to left hand side, we run out of an XPath stack page (4K), we need slightly more than 2K/8 = 256 nodes on 32-bit, 128 nodes on 64-bit
+	size_t count = sizeof(void*) == 4 ? 300 : 150;
 
-	xpath_query q(STR("a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|(a|a)))))))))))))))))))"));
+	for (size_t i = 0; i < count; ++i)
+		doc.first_child().append_child(STR("a"));
+
+	xpath_query q(STR("a|a"));
+
+	test_runner::_memory_fail_threshold = 1;
 
 	CHECK_ALLOC_FAIL(CHECK(q.evaluate_node_set(doc.child(STR("node"))).empty()));
 }
-*/
+
+TEST_XML(xpath_out_of_memory_evaluate_union_hash, "<node />")
+{
+	// left hand side: size * sizeof(xpath_node) (8 on 32-bit, 16 on 64-bit)
+	// right hand side: same
+	// hash table: size * 1.5 * sizeof(void*)
+	// to make sure that when we append right hand side to left hand side, we do *not* run out of an XPath stack page (4K), we need slightly less than 2K/8 = 256 nodes on 32-bit, 128 nodes on 64-bit
+	size_t count = sizeof(void*) == 4 ? 200 : 100;
+
+	for (size_t i = 0; i < count; ++i)
+		doc.first_child().append_child(STR("a"));
+
+	xpath_query q(STR("a|a"));
+
+	test_runner::_memory_fail_threshold = 1;
+
+	CHECK_ALLOC_FAIL(CHECK(q.evaluate_node_set(doc.child(STR("node"))).empty()));
+}
+
 
 TEST_XML(xpath_out_of_memory_evaluate_predicate, "<node><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/><a/></node>")
 {