diff --git a/include/pocketpy/compiler/lexer.hpp b/include/pocketpy/compiler/lexer.hpp
index 92f02c2b..f3141c94 100644
--- a/include/pocketpy/compiler/lexer.hpp
+++ b/include/pocketpy/compiler/lexer.hpp
@@ -25,9 +25,11 @@ constexpr const char* kTokens[] = {
     "**", "=", ">", "<", "..", "...", "->", "@", "==", "!=", ">=", "<=",
     "++", "--", "~",
     /** KW_BEGIN **/
-    "class", "import", "as", "def", "lambda", "pass", "del", "from", "with", "yield",
-    "None", "in", "is", "and", "or", "not", "True", "False", "global", "try", "except", "finally",
-    "while", "for", "if", "elif", "else", "break", "continue", "return", "assert", "raise"
+    // NOTE: These keywords should be sorted in ascending order!!
+    "False", "None", "True", "and", "as", "assert", "break", "class", "continue",
+    "def", "del", "elif", "else", "except", "finally", "for", "from", "global",
+    "if", "import", "in", "is", "lambda", "not", "or", "pass", "raise", "return",
+    "try", "while", "with", "yield",
 };
 // clang-format on
 
@@ -50,12 +52,6 @@ constexpr TokenIndex TK(const char token[]) {
 constexpr inline bool is_raw_string_used(TokenIndex t) { return t == TK("@id") || t == TK("@long"); }
 
 #define TK_STR(t) kTokens[t]
-const small_map<std::string_view, TokenIndex> kTokenKwMap = []() {
-    small_map<std::string_view, TokenIndex> map;
-    for(int k = TK("class"); k < kTokenCount; k++)
-        map.insert(kTokens[k], k);
-    return map;
-}();
 
 struct Token {
     TokenIndex type;
diff --git a/src/compiler/lexer.cpp b/src/compiler/lexer.cpp
index e14d1eed..66c18bbe 100644
--- a/src/compiler/lexer.cpp
+++ b/src/compiler/lexer.cpp
@@ -153,9 +153,13 @@ int Lexer::eat_name() {
         return 0;
     }
 
-    auto it = kTokenKwMap.try_get(name);
-    if(it != nullptr) {
-        add_token(*it);
+    const auto KW_BEGIN = kTokens + TK("False");
+    const auto KW_END = kTokens + kTokenCount;
+
+    auto it = std::lower_bound(KW_BEGIN, KW_END, name);
+    if(it != KW_END) {
+        assert(*it == name);
+        add_token(it - kTokens);
     } else {
         add_token(TK("@id"));
     }