Index: src/basemath/alglin2.c
===================================================================
--- src/basemath/alglin2.c	(revision 9574)
+++ src/basemath/alglin2.c	(working copy)
@@ -3082,11 +3082,9 @@
     for (i=h[j]; i; i--)
     {
       a = gcoeff(A,i,j);
-      if (!signe(a)) continue;
-
       k = c[i];
-      ZV_elem(a,gcoeff(A,i,k), A,B, j,k);
-      ZM_reduce(A,B, i,k);
+      if (signe(a)) ZV_elem(a,gcoeff(A,i,k), A,B, j,k);
+      ZM_reduce(A,B, i,k); /* ensure non-negative entries, even if a = 0 */
       if (low_stack(lim, stack_lim(av1,1)))
       {
         if (DEBUGMEM>1) pari_warn(warnmem,"hnfall[3], j = %ld", j);