diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 2c51d64972..0711d82fe5 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -3144,7 +3144,7 @@ static char *sub_grow_buf(char **new_start, int *new_start_len, int needed_len)
     // substitution into (and some extra space to avoid
     // too many calls to xmalloc()/free()).
     *new_start_len = needed_len + 50;
-    *new_start = xmalloc((size_t)(*new_start_len));
+    *new_start = xcalloc(1, (size_t)(*new_start_len));
     **new_start = NUL;
     new_end = *new_start;
   } else {
@@ -3154,8 +3154,11 @@ static char *sub_grow_buf(char **new_start, int *new_start_len, int needed_len)
     size_t len = strlen(*new_start);
     needed_len += (int)len;
     if (needed_len > *new_start_len) {
+      size_t prev_new_start_len = (size_t)(*new_start_len);
       *new_start_len = needed_len + 50;
+      size_t added_len = (size_t)(*new_start_len) - prev_new_start_len;
       *new_start = xrealloc(*new_start, (size_t)(*new_start_len));
+      memset(*new_start + prev_new_start_len, 0, added_len);
     }
     new_end = *new_start + len;
   }
diff --git a/test/old/testdir/crash/poc_ex_substitute b/test/old/testdir/crash/poc_ex_substitute
new file mode 100644
index 0000000000..bcf1286512
Binary files /dev/null and b/test/old/testdir/crash/poc_ex_substitute differ
diff --git a/test/old/testdir/test_crash.vim b/test/old/testdir/test_crash.vim
index 1d4f435e4d..49e712a901 100644
--- a/test/old/testdir/test_crash.vim
+++ b/test/old/testdir/test_crash.vim
@@ -117,7 +117,7 @@ func Test_crash1_2()
   " The following used to crash Vim
   let opts = #{cmd: 'sh'}
   let vim  = GetVimProg()
-  let result = 'X_crash1_1_result.txt'
+  let result = 'X_crash1_2_result.txt'
 
   let buf = RunVimInTerminal('sh', opts)
 
@@ -149,6 +149,17 @@ func Test_crash1_2()
     \ ' ; echo "crash 4: [OK]" >> '.. result .. "\<cr>")
   call TermWait(buf, 150)
 
+  let file = 'crash/poc_ex_substitute'
+  let cmn_args = "%s -u NONE -i NONE -n -e -s -S %s -c ':qa!'"
+  let args = printf(cmn_args, vim, file)
+  " just make sure it runs, we don't care about the resulting echo
+  call term_sendkeys(buf, args .. "\<cr>")
+  " There is no output generated in Github CI for the asan clang build.
+  " so just skip generating the ouput.
+  " call term_sendkeys(buf, args ..
+  "   \ ' &&  echo "crash 5: [OK]" >> '.. result .. "\<cr>")
+  call TermWait(buf, 150)
+
   " clean up
   exe buf .. "bw!"