diff --git a/src/nvim/spellfile.c b/src/nvim/spellfile.c
index 466aaf4578..f934b9f94e 100644
--- a/src/nvim/spellfile.c
+++ b/src/nvim/spellfile.c
@@ -1553,7 +1553,9 @@ static int set_sofo(slang_T *lp, const char *from, const char *to)
   lp->sl_sofo = true;
 
   // First count the number of items for each list.  Temporarily use
-  // sl_sal_first[] for this.
+  // sl_sal_first[] for this.  Reset it first: a preceding SN_SAL section
+  // may have set the entries to -1 via set_sal_first().
+  memset(lp->sl_sal_first, 0, sizeof(salfirst_T) * 256);
   for (p = from, s = to; *p != NUL && *s != NUL;) {
     const int c = mb_cptr2char_adv(&p);
     s += utf_ptr2len(s);
diff --git a/test/old/testdir/test_spellfile.vim b/test/old/testdir/test_spellfile.vim
index 0938e4cfd1..9919821a6f 100644
--- a/test/old/testdir/test_spellfile.vim
+++ b/test/old/testdir/test_spellfile.vim
@@ -322,6 +322,11 @@ func Test_spellfile_format_error()
   " SN_SOFO: multi-byte characters in sofofrom and sofoto
   call Spellfile_Test(0z0600000000080002CF810002CF82FF000000000000000000000000, '')
 
+  " SN_SAL (empty) followed by SN_SOFO with two multi-byte 'from' characters
+  " sharing the same low byte.  A preceding SN_SAL poisons sl_sal_first[], so
+  " without a reset set_sofo() under-counts and writes out of bounds.
+  call Spellfile_Test(0z05000000000300000006000000000A0004CAABCEAB00024142FF000000000000000000000000, '')
+
   " SN_COMPOUND: compmax is less than 2
   call Spellfile_Test(0z08000000000101, 'E759:')