diff --git a/gmp.lisp b/gmp.lisp
index f8f23f4..a003d58 100644
--- a/gmp.lisp
+++ b/gmp.lisp
@@ -420,9 +420,11 @@ be (1+ COUNT)."
                    (error "Size of result exceeds maxim bignum length")) into checks
         collect `(,res (%allocate-bignum (1+ ,size)))
           into resallocs
-        collect `(setf ,res (if (minusp (slot ,gres 'mp_size)) ; check for negative result
-                                (gmp-z-to-bignum-neg (slot ,gres 'mp_d) ,res ,size)
-                                (gmp-z-to-bignum (slot ,gres 'mp_d) ,res ,size)))
+        collect `(setf ,res (cond ((minusp (slot ,gres 'mp_size))
+                                   (%bignum-set ,res ,size sb-bignum::all-ones-digit)
+                                   (gmp-z-to-bignum-neg (slot ,gres 'mp_d) ,res ,size))
+                                  (t
+                                   (gmp-z-to-bignum (slot ,gres 'mp_d) ,res ,size))))
           into copylimbs
         collect `(__gmpz_clear (addr ,gres)) into clears
         collect res into results