diff --git a/py/mpz.c b/py/mpz.c
index 1dd177b..1861840 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -308,9 +308,13 @@
         carryi >>= DIG_SIZE;
     }
 
-    if (0 != carryi) {
-        *idig++ = carryi;
-    }
+    // At least one of j,k must be negative so the above for-loop runs at least
+    // once.  For carryi to be non-zero here it must be equal to 1 at the end of
+    // each iteration of the loop.  So the accumulation of carryi must overflow
+    // each time, ie carryi += 0xff..ff.  So carryj|carryk must be 0 in the
+    // DIG_MASK bits on each iteration.  But considering all cases of signs of
+    // j,k one sees that this is not possible.
+    assert(carryi == 0);
 
     return mpn_remove_trailing_zeros(oidig, idig);
 }
@@ -334,9 +338,8 @@
         carryi >>= DIG_SIZE;
     }
 
-    if (0 != carryi) {
-        *idig++ = carryi;
-    }
+    // See comment in above mpn_or_neg for why carryi must be 0.
+    assert(carryi == 0);
 
     return mpn_remove_trailing_zeros(oidig, idig);
 }