diff --git a/internal/numeric.h b/internal/numeric.h
index 75181a7f16..d3905f048c 100644
--- a/internal/numeric.h
+++ b/internal/numeric.h
@@ -164,6 +164,11 @@ union rb_int128 {
 };
 typedef union rb_int128 rb_int128_t;
 
+union uint128_int128_conversion {
+    rb_uint128_t uint128;
+    rb_int128_t int128;
+};
+
 // Conversion functions for 128-bit integers:
 rb_uint128_t rb_numeric_to_uint128(VALUE x);
 rb_int128_t rb_numeric_to_int128(VALUE x);
diff --git a/io_buffer.c b/io_buffer.c
index b81527ff71..55f1933194 100644
--- a/io_buffer.c
+++ b/io_buffer.c
@@ -1928,11 +1928,6 @@ ruby_swap128_uint(rb_uint128_t x)
     return result;
 }
 
-union uint128_int128_conversion {
-    rb_uint128_t uint128;
-    rb_int128_t int128;
-};
-
 static inline rb_int128_t
 ruby_swap128_int(rb_int128_t x)
 {
diff --git a/numeric.c b/numeric.c
index d9e837644f..1942a6164f 100644
--- a/numeric.c
+++ b/numeric.c
@@ -3638,7 +3638,10 @@ rb_int128_to_numeric(rb_int128_t n)
     }
     else {
         // Positive value
-        return rb_uint128_to_numeric(*(rb_uint128_t*)&n);
+        union uint128_int128_conversion conversion = {
+            .int128 = n
+        };
+        return rb_uint128_to_numeric(conversion.uint128);
     }
 #endif
 }