diff --git a/src/backend/mir_interpreter/handlers/boxes_array.rs b/src/backend/mir_interpreter/handlers/boxes_array.rs
index 3226d242..a4c3571d 100644
--- a/src/backend/mir_interpreter/handlers/boxes_array.rs
+++ b/src/backend/mir_interpreter/handlers/boxes_array.rs
@@ -26,7 +26,7 @@ pub(super) fn try_handle_array_box(
                 }
                 "push" => {
                     this.validate_args_exact("push", args, 1)?;
-                    let val = this.reg_load(args[0])?.to_nyash_box();
+                    let val = this.load_as_box(args[0])?;
                     let _ = ab.push(val);
                     this.write_void(dst);
                     return Ok(true);
@@ -44,15 +44,15 @@ pub(super) fn try_handle_array_box(
                 }
                 "get" => {
                     this.validate_args_exact("get", args, 1)?;
-                    let idx = this.reg_load(args[0])?.to_nyash_box();
+                    let idx = this.load_as_box(args[0])?;
                     let ret = ab.get(idx);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);
                 }
                 "set" => {
                     this.validate_args_exact("set", args, 2)?;
-                    let idx = this.reg_load(args[0])?.to_nyash_box();
-                    let val = this.reg_load(args[1])?.to_nyash_box();
+                    let idx = this.load_as_box(args[0])?;
+                    let val = this.load_as_box(args[1])?;
                     let _ = ab.set(idx, val);
                     this.write_void(dst);
                     return Ok(true);
diff --git a/src/backend/mir_interpreter/handlers/boxes_map.rs b/src/backend/mir_interpreter/handlers/boxes_map.rs
index c96f1893..c93f95dc 100644
--- a/src/backend/mir_interpreter/handlers/boxes_map.rs
+++ b/src/backend/mir_interpreter/handlers/boxes_map.rs
@@ -33,7 +33,7 @@ pub(super) fn try_handle_map_box(
                         this.write_result(dst, VMValue::String("[map/bad-key] field name must be string".to_string()));
                         return Ok(true);
                     }
-                    let k = k_vm.to_nyash_box();
+                    let k = this.load_as_box(args[0])?;
                     let ret = mb.get(k);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);
@@ -45,8 +45,8 @@ pub(super) fn try_handle_map_box(
                         this.write_result(dst, VMValue::String("[map/bad-key] field name must be string".to_string()));
                         return Ok(true);
                     }
-                    let k = k_vm.to_nyash_box();
-                    let v = this.reg_load(args[1])?.to_nyash_box();
+                    let k = this.load_as_box(args[0])?;
+                    let v = this.load_as_box(args[1])?;
                     let ret = mb.set(k, v);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);
@@ -58,8 +58,8 @@ pub(super) fn try_handle_map_box(
                         this.write_result(dst, VMValue::String("[map/bad-key] key must be string".to_string()));
                         return Ok(true);
                     }
-                    let k = k_vm.to_nyash_box();
-                    let v = this.reg_load(args[1])?.to_nyash_box();
+                    let k = this.load_as_box(args[0])?;
+                    let v = this.load_as_box(args[1])?;
                     let ret = mb.set(k, v);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);
@@ -71,14 +71,14 @@ pub(super) fn try_handle_map_box(
                         this.write_result(dst, VMValue::String("[map/bad-key] key must be string".to_string()));
                         return Ok(true);
                     }
-                    let k = k_vm.to_nyash_box();
+                    let k = this.load_as_box(args[0])?;
                     let ret = mb.get(k);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);
                 }
                 "has" => {
                     this.validate_args_exact("MapBox.has", args, 1)?;
-                    let k = this.reg_load(args[0])?.to_nyash_box();
+                    let k = this.load_as_box(args[0])?;
                     let ret = mb.has(k);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);
@@ -90,7 +90,7 @@ pub(super) fn try_handle_map_box(
                         this.write_result(dst, VMValue::String("[map/bad-key] key must be string".to_string()));
                         return Ok(true);
                     }
-                    let k = k_vm.to_nyash_box();
+                    let k = this.load_as_box(args[0])?;
                     let ret = mb.delete(k);
                     this.write_result(dst, VMValue::from_nyash_box(ret));
                     return Ok(true);