diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index a54056164e..9d6f7d972c 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -5321,6 +5321,7 @@ static void reduce_string(typval_T *argvars, typval_T *expr, typval_T *rettv)
   for (; *p != NUL; p += len) {
     typval_T argv[3];
     argv[0] = *rettv;
+    rettv->v_type = VAR_UNKNOWN;
     len = utfc_ptr2len(p);
     argv[1] = (typval_T){
       .v_type = VAR_STRING,
diff --git a/test/old/testdir/test_listdict.vim b/test/old/testdir/test_listdict.vim
index 78d3a19cfb..f40a4290cc 100644
--- a/test/old/testdir/test_listdict.vim
+++ b/test/old/testdir/test_listdict.vim
@@ -1066,6 +1066,14 @@ func Test_reduce()
   " call assert_fails('echo reduce([1], test_null_function())', 'E1132:')
   " Nvim doesn't have null partials
   " call assert_fails('echo reduce([1], test_null_partial())', 'E1132:')
+
+  " did cause double free
+  function! OuterReduce()
+    vim9 echo reduce('ab', 42)
+  endfunction
+  "call assert_fails('call OuterReduce()', 'E1024:')
+  call assert_fails("echo reduce('ab', 'NoSuchFunc')", 'E117:')
+  delfunc OuterReduce
 endfunc
 
 " splitting a string to a List using split()