diff --git a/c/external.c b/c/external.c
index 808647c..343d899 100644
--- a/c/external.c
+++ b/c/external.c
@@ -835,7 +835,7 @@ s48_extract_unsigned_integer(s48_value value)
 	  (! S48_FIXNUM_P(boxed_high)) ||
 	  (high >  0xFFFF)) 
 	s48_raise_argtype_error(value);
-      else return (- (((- high) << 16) - low));
+      else return ((((unsigned long) high) << 16) + low);
     }
   }
 }