diff --git a/README.md b/README.md
index 4a85ce8..387d771 100644
--- a/README.md
+++ b/README.md
@@ -62,6 +62,7 @@ $ sh boot.sh
   - 函数名字本身建议仅用于直接调用，其它行为可视为 UB。
 - 数组只支持一维数组，且数组的元素不能是指针类型。
 - 全局变量不能是指针类型。
+- 整数和字符字面量的类型是 `int`，字符串字面量的类型是 `char*`
 
 ### 支持的流程控制
 
diff --git a/boot.c b/boot.c
index f19f1b2..6ff3bff 100644
--- a/boot.c
+++ b/boot.c
@@ -1080,6 +1080,10 @@ int parse_expr() {
 }
 
 void parse_local_variable(int type) {
+    if (type == TYPE_VOID) {
+        eprintf("local variable of void type is not supported\n");
+        exit(1);
+    }
     expect_token(TOKEN_ID);
     int id = token_data;
     next_token();
@@ -1252,7 +1256,19 @@ void parse_function(const char* name) {
             expect_token(TOKEN_PAREN_RIGHT);
             break;
         }
+        if (token_type == TOKEN_VOID) {
+            if (arg != 0) {
+                eprintf("void should be the only argument\n");
+                exit(1);
+            }
+            expect_token(TOKEN_PAREN_RIGHT);
+            break;
+        }
         int arg_type = parse_type();
+        if (arg_type < 0 || arg_type == TYPE_VOID) {
+            eprintf("unexpected a non-void argument type");
+            exit(1);
+        }
         expect_token(TOKEN_ID);
         int arg_name = token_data;
         next_token();
@@ -1322,6 +1338,10 @@ void parse_function(const char* name) {
 }
 
 void parse_global_variable(int id, const char* name, int type) {
+    if (type == TYPE_VOID) {
+        eprintf("global variable of void type is not supported\n");
+        exit(1);
+    }
     if (type & TYPE_PTR_MASK) {
         eprintf("global variable of pointer is not supported\n");
         exit(1);
@@ -1353,7 +1373,7 @@ void parse_global_variable(int id, const char* name, int type) {
 void parse_global_declaration() {
     int type = parse_type();
     if (type < 0) {
-        eprintf("unexpected token: %d\n", token_type);
+        eprintf("expecting type for global declaration\n");
         exit(1);
     }
     expect_token(TOKEN_ID);