diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 4127dacaef..24ebfadb21 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -5412,8 +5412,6 @@ static void filter_map_dict(dict_T *d, filtermap_T filtermap, const char *func_n
     d_ret = rettv->vval.v_dict;
   }
 
-  vimvars[VV_KEY].vv_type = VAR_STRING;
-
   const VarLockStatus prev_lock = d->dv_lock;
   if (d->dv_lock == VAR_UNLOCKED) {
     d->dv_lock = VAR_LOCKED;
@@ -5425,11 +5423,11 @@ static void filter_map_dict(dict_T *d, filtermap_T filtermap, const char *func_n
             || var_check_ro(di->di_flags, arg_errmsg, TV_TRANSLATE))) {
       break;
     }
-    vimvars[VV_KEY].vv_str = xstrdup(di->di_key);
+    set_vim_var_string(VV_KEY, di->di_key, -1);
     typval_T newtv;
     bool rem;
     int r = filter_map_one(&di->di_tv, expr, filtermap, &newtv, &rem);
-    tv_clear(&vimvars[VV_KEY].vv_tv);
+    tv_clear(get_vim_var_tv(VV_KEY));
     if (r == FAIL || did_emsg) {
       tv_clear(&newtv);
       break;
@@ -5496,7 +5494,7 @@ static void filter_map_blob(blob_T *blob_arg, filtermap_T filtermap, typval_T *e
       .v_lock = VAR_UNLOCKED,
       .vval.v_number = val,
     };
-    vimvars[VV_KEY].vv_nr = idx;
+    set_vim_var_nr(VV_KEY, idx);
     typval_T newtv;
     bool rem;
     if (filter_map_one(&tv, expr, filtermap, &newtv, &rem) == FAIL
@@ -5614,7 +5612,7 @@ static void filter_map_list(list_T *l, filtermap_T filtermap, const char *func_n
         && value_check_lock(TV_LIST_ITEM_TV(li)->v_lock, arg_errmsg, TV_TRANSLATE)) {
       break;
     }
-    vimvars[VV_KEY].vv_nr = idx;
+    set_vim_var_nr(VV_KEY, idx);
     typval_T newtv;
     bool rem;
     if (filter_map_one(TV_LIST_ITEM_TV(li), expr, filtermap, &newtv, &rem) == FAIL) {
@@ -5684,15 +5682,16 @@ static void filter_map(typval_T *argvars, typval_T *rettv, filtermap_T filtermap
   }
 
   typval_T save_val;
+  typval_T save_key;
+
   prepare_vimvar(VV_VAL, &save_val);
+  prepare_vimvar(VV_KEY, &save_key);
 
   // We reset "did_emsg" to be able to detect whether an error
   // occurred during evaluation of the expression.
   int save_did_emsg = did_emsg;
   did_emsg = false;
 
-  typval_T save_key;
-  prepare_vimvar(VV_KEY, &save_key);
   if (argvars[0].v_type == VAR_DICT) {
     filter_map_dict(argvars[0].vval.v_dict, filtermap, func_name,
                     arg_errmsg, expr, rettv);