diff --git a/ext/yarp/api_pack.c b/ext/yarp/api_pack.c
index 05f9dc42c08..7f0440970e3 100644
--- a/ext/yarp/api_pack.c
+++ b/ext/yarp/api_pack.c
@@ -9,6 +9,16 @@ static VALUE v3_2_0_symbol;
 static VALUE pack_symbol;
 static VALUE unpack_symbol;
 
+#if SIZEOF_UINT64_T == SIZEOF_LONG_LONG
+# define UINT64T2NUM(x) ULL2NUM(x)
+# define NUM2UINT64T(x) (uint64_t)NUM2ULL(x)
+#elif SIZEOF_UINT64_T == SIZEOF_LONG
+# define UINT64T2NUM(x) ULONG2NUM(x)
+# define NUM2UINT64T(x) (uint64_t)NUM2ULONG(x)
+#else
+// error No uint64_t conversion
+#endif
+
 static VALUE
 pack_type_to_symbol(yp_pack_type type) {
     switch (type) {
@@ -221,7 +231,7 @@ pack_parse(VALUE self, VALUE version_symbol, VALUE variant_symbol, VALUE format_
                                     pack_endian_to_symbol(endian),
                                     pack_size_to_symbol(size),
                                     pack_length_type_to_symbol(length_type),
-                                    (long) LONG2NUM(length) };
+                                    UINT64T2NUM(length) };
 
         rb_ary_push(directives_array, rb_class_new_instance(9, directive_args, rb_cYARPPackDirective));
     }