diff --git a/HISTORY b/HISTORY
index 2f582c9..5ffe3c6 100755
--- a/HISTORY
+++ b/HISTORY
@@ -1,5 +1,8 @@
+Jun. 28 2021
+Improved error prompting of assigning to reserved word
+
 Apr. 9 2020
-Improved incomplete IF structure error prompting
+Improved error prompting of incomplete IF structure
 
 Mar. 16 2019
 Fixed a memory leak when reassigning an array element with string, thanks to Jacques Diederik for pointing it out
diff --git a/core/my_basic.c b/core/my_basic.c
index 47450b5..4b10028 100755
--- a/core/my_basic.c
+++ b/core/my_basic.c
@@ -271,6 +271,7 @@ MBCONST static const char* const _ERR_DESC[] = {
 	"Too many dimensions",
 	"Rank out of bound",
 	"Invalid identifier usage",
+	"Cannot assign to reserved word",
 	"Duplicate identifier",
 	"Incomplete structure",
 	"Label not exists",
@@ -15426,7 +15427,7 @@ static int _core_let(mb_interpreter_t* s, void** l) {
 			goto _exit;
 	} else if(obj->type == _DT_VAR) {
 		if(_IS_ME(obj->data.variable)) {
-			_handle_error_on_obj(s, SE_RN_INVALID_ID_USAGE, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
+			_handle_error_on_obj(s, SE_RN_CANNOT_ASSIGN_TO_RESERVED_WORD, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
 		} else {
 			evar = obj->data.variable;
 			var = _search_var_in_scope_chain(s, obj->data.variable, 0);
@@ -15435,11 +15436,11 @@ static int _core_let(mb_interpreter_t* s, void** l) {
 			if(evar && evar->pathing == _PATHING_UPVALUE) evar = 0;
 #endif /* MB_ENABLE_CLASS */
 			if(var == _OBJ_BOOL_TRUE->data.variable || var == _OBJ_BOOL_FALSE->data.variable) {
-				_handle_error_on_obj(s, SE_RN_INVALID_ID_USAGE, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
+				_handle_error_on_obj(s, SE_RN_CANNOT_ASSIGN_TO_RESERVED_WORD, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
 			}
 		}
 	} else {
-		_handle_error_on_obj(s, SE_RN_INVALID_ID_USAGE, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
+		_handle_error_on_obj(s, SE_RN_CANNOT_ASSIGN_TO_RESERVED_WORD, s->source_file, DON(ast), MB_FUNC_ERR, _exit, result);
 	}
 
 	ast = ast->next;
diff --git a/core/my_basic.h b/core/my_basic.h
index 8c6eb75..c63c45b 100755
--- a/core/my_basic.h
+++ b/core/my_basic.h
@@ -425,6 +425,7 @@ typedef enum mb_error_e {
 	SE_RN_TOO_MANY_DIMENSIONS,
 	SE_RN_RANK_OUT_OF_BOUND,
 	SE_RN_INVALID_ID_USAGE,
+	SE_RN_CANNOT_ASSIGN_TO_RESERVED_WORD,
 	SE_RN_DUPLICATE_ID,
 	SE_RN_INCOMPLETE_STRUCTURE,
 	SE_RN_LABEL_NOT_EXISTS,