diff --git a/src/jrd/btr.cpp b/src/jrd/btr.cpp
index 43a8cc794a0..61fb180a37f 100644
--- a/src/jrd/btr.cpp
+++ b/src/jrd/btr.cpp
@@ -233,10 +233,14 @@ namespace
 
 			if (traNumber && rpt->getTransaction() != traNumber)
 			{
-				const auto backId = rpt->irt_backslot;
-				fb_assert(backId < root->irt_count);
+				const auto backSlot = root->getBackSlot(rpt);
+				if (!backSlot)
+				{
+					CCH_unwind(tdbb, false);
+					BUGCHECK(204);		// msg 204 index inconsistent
+				}
 
-				return &root->irt_rpt[backId];
+				return backSlot;
 			}
 		}
 
@@ -3187,10 +3191,15 @@ static ModifyIrtRepeatValue modifyIrtRepeat(thread_db* tdbb, index_root_page::ir
 			if (state != tra_committed && state != tra_dead)
 				return ModifyIrtRepeatValue::Skip;
 
-			CCH_MARK(tdbb, window);
 			const auto backId = irt_desc->irt_backslot;
-			fb_assert(backId < root->irt_count);
-			const auto backIrtDesc = &root->irt_rpt[backId];
+			const auto backIrtDesc = root->getBackSlot(irt_desc);
+			if (!backIrtDesc)
+			{
+				CCH_RELEASE(tdbb, window);
+				BUGCHECK(204);		// msg 204 index inconsistent
+			}
+
+			CCH_MARK(tdbb, window);
 
 			if (state == tra_committed)
 			{
diff --git a/src/jrd/ods.h b/src/jrd/ods.h
index 76e0b4f16bf..41cfc61f77a 100644
--- a/src/jrd/ods.h
+++ b/src/jrd/ods.h
@@ -449,6 +449,12 @@ struct index_root_page
 		void setEmpty();
 	} irt_rpt[1];
 
+	// Return the backing slot referenced by a migrating index slot,
+	// or NULL when the slot is not in the irt_migrate state or the
+	// stored slot number points outside this page
+	const irt_repeat* getBackSlot(const irt_repeat* rpt) const;
+	irt_repeat* getBackSlot(const irt_repeat* rpt);
+
 	static_assert(sizeof(struct irt_repeat) == 24, "struct irt_repeat size mismatch");
 	static_assert(offsetof(struct irt_repeat, irt_transaction) == 0, "irt_transaction offset mismatch");
 	static_assert(offsetof(struct irt_repeat, irt_page_num) == 8, "irt_root offset mismatch");
@@ -775,6 +781,22 @@ inline void index_root_page::irt_repeat::setMigrate(const Jrd::PageNumber& rootP
 	setState(irt_migrate);
 }
 
+inline const index_root_page::irt_repeat* index_root_page::getBackSlot(const irt_repeat* rpt) const
+{
+	fb_assert(rpt->getState() == irt_migrate);
+
+	if (rpt->getState() != irt_migrate || rpt->irt_backslot >= irt_count)
+		return nullptr;
+
+	return &irt_rpt[rpt->irt_backslot];
+}
+
+inline index_root_page::irt_repeat* index_root_page::getBackSlot(const irt_repeat* rpt)
+{
+	return const_cast<irt_repeat*>(
+		static_cast<const index_root_page*>(this)->getBackSlot(rpt));
+}
+
 inline constexpr int STUFF_COUNT		= 4;
 
 inline constexpr ULONG END_LEVEL		= ~0;
diff --git a/src/jrd/tests/IndexRootPageTest.cpp b/src/jrd/tests/IndexRootPageTest.cpp
new file mode 100644
index 00000000000..96172203355
--- /dev/null
+++ b/src/jrd/tests/IndexRootPageTest.cpp
@@ -0,0 +1,91 @@
+#include "firebird.h"
+#include "boost/test/unit_test.hpp"
+#include "../jrd/ods.h"
+
+using namespace Firebird;
+using namespace Jrd;
+using namespace Ods;
+
+BOOST_AUTO_TEST_SUITE(EngineSuite)
+BOOST_AUTO_TEST_SUITE(IndexRootPageSuite)
+
+
+BOOST_AUTO_TEST_SUITE(BackSlotTests)
+
+// Index root page image with room for 16 slots
+union RootPageImage
+{
+	index_root_page page;
+	UCHAR bytes[sizeof(index_root_page) + 15 * sizeof(index_root_page::irt_repeat)];
+};
+
+// Zero-filled image, so every test case starts from a clean page
+static index_root_page* makeRootPage(unsigned count)
+{
+	static RootPageImage image;
+
+	memset(&image, 0, sizeof(image));
+	image.page.irt_relation = 1;
+	image.page.irt_count = count;
+	return &image.page;
+}
+
+// Bring a slot into the irt_migrate state through the regular state
+// transitions, ending with the given backing slot number
+static index_root_page::irt_repeat* makeMigratedSlot(index_root_page::irt_repeat* slot, USHORT backSlot)
+{
+	slot->setInProgress(12);
+	slot->setInProgress(PageNumber(1, 77), 12);
+	slot->setKill();
+	slot->setRollback(12);
+	slot->setMigrate(PageNumber(1, 99), 12, backSlot);
+	return slot;
+}
+
+BOOST_AUTO_TEST_CASE(BackSlotInRange)
+{
+	auto root = makeRootPage(4);
+
+	makeMigratedSlot(&root->irt_rpt[1], 2);
+	BOOST_TEST(root->getBackSlot(&root->irt_rpt[1]) == &root->irt_rpt[2]);
+
+	root->irt_rpt[1].setMigrate(PageNumber(1, 99), 12, 3);		// last valid slot
+	BOOST_TEST(root->getBackSlot(&root->irt_rpt[1]) == &root->irt_rpt[3]);
+}
+
+BOOST_AUTO_TEST_CASE(BackSlotOutOfRange)
+{
+	auto root = makeRootPage(4);
+
+	makeMigratedSlot(&root->irt_rpt[0], 4);		// == irt_count
+	BOOST_TEST(root->getBackSlot(&root->irt_rpt[0]) == nullptr);
+
+	root->irt_rpt[0].setMigrate(PageNumber(1, 99), 12, 0xFFFF);
+	BOOST_TEST(root->getBackSlot(&root->irt_rpt[0]) == nullptr);
+
+	root->irt_count = 0;		// no slot may resolve anymore
+	BOOST_TEST(root->getBackSlot(&root->irt_rpt[0]) == nullptr);
+}
+
+BOOST_AUTO_TEST_CASE(BackSlotNotMigrating)
+{
+	auto root = makeRootPage(4);
+
+	// slot in the regular working state: the backing slot number is
+	// meaningless and must not resolve even when in range
+	auto slot = &root->irt_rpt[1];
+	slot->setInProgress(12);
+	slot->setInProgress(PageNumber(1, 77), 12);
+	slot->setKill();
+	slot->setRollback(12);
+	slot->setNormal();
+	slot->irt_backslot = 2;
+
+	BOOST_TEST(root->getBackSlot(slot) == nullptr);
+}
+
+BOOST_AUTO_TEST_SUITE_END()	// BackSlotTests
+
+
+BOOST_AUTO_TEST_SUITE_END()	// IndexRootPageSuite
+BOOST_AUTO_TEST_SUITE_END()	// EngineSuite