diff --git a/nac3core/irrt/irrt/int_types.hpp b/nac3core/irrt/irrt/int_types.hpp
index 87900961..656a060e 100644
--- a/nac3core/irrt/irrt/int_types.hpp
+++ b/nac3core/irrt/irrt/int_types.hpp
@@ -1,13 +1,22 @@
 #pragma once
 
+#if __STDC_VERSION__ >= 202000
 using int8_t = _BitInt(8);
 using uint8_t = unsigned _BitInt(8);
 using int32_t = _BitInt(32);
 using uint32_t = unsigned _BitInt(32);
 using int64_t = _BitInt(64);
 using uint64_t = unsigned _BitInt(64);
+#else
+using int8_t = _ExtInt(8);
+using uint8_t = unsigned _ExtInt(8);
+using int32_t = _ExtInt(32);
+using uint32_t = unsigned _ExtInt(32);
+using int64_t = _ExtInt(64);
+using uint64_t = unsigned _ExtInt(64);
+#endif
 
 // NDArray indices are always `uint32_t`.
 using NDIndex = uint32_t;
 // The type of an index or a value describing the length of a range/slice is always `int32_t`.
-using SliceIndex = int32_t;
\ No newline at end of file
+using SliceIndex = int32_t;