diff --git a/kalcy/include/kalcy/error.hpp b/kalcy/include/kalcy/error.hpp
index e6775e7..4ee3436 100644
--- a/kalcy/include/kalcy/error.hpp
+++ b/kalcy/include/kalcy/error.hpp
@@ -21,6 +21,13 @@ struct InternalError : Error {
 	using Error::Error;
 };
 
+///
+/// \brief Unrecognized token.
+///
+struct UnrecognizedToken : Error {
+	explicit UnrecognizedToken(Token token);
+};
+
 ///
 /// \brief Parse error.
 ///
diff --git a/kalcy/src/error.cpp b/kalcy/src/error.cpp
index 3b63826..769cf03 100644
--- a/kalcy/src/error.cpp
+++ b/kalcy/src/error.cpp
@@ -11,6 +11,8 @@ auto Error::build_highlight(char const higlight) const -> std::string {
 	return ret.str();
 }
 
+UnrecognizedToken::UnrecognizedToken(Token const token) : Error(token, std::format("unrecognized token: '{}'", token.lexeme)) {}
+
 ParseError::ParseError(Token const token) : Error(token, "parse error") {}
 
 ParseError::ParseError(Token const token, Token::Type const expected)
diff --git a/kalcy/src/scanner.cpp b/kalcy/src/scanner.cpp
index 20382d9..f821123 100644
--- a/kalcy/src/scanner.cpp
+++ b/kalcy/src/scanner.cpp
@@ -1,3 +1,4 @@
+#include <kalcy/error.hpp>
 #include <kalcy/scanner.hpp>
 #include <cassert>
 #include <cctype>
@@ -28,7 +29,7 @@ auto Scanner::next() -> Token {
 	if (match_number(out)) { return out; }
 	if (match_identifier(out)) { return out; }
 
-	return {};
+	throw UnrecognizedToken{make_token(Token::Type::eNone, {&m_text.front(), 1})};
 }
 
 auto Scanner::match_single(Token& out) -> bool {
diff --git a/tests/tests/test_parser.cpp b/tests/tests/test_parser.cpp
index 1811308..d6482ca 100644
--- a/tests/tests/test_parser.cpp
+++ b/tests/tests/test_parser.cpp
@@ -97,6 +97,28 @@ ADD_TEST(ParseErrorOnExtraneous) {
 	EXPECT(did_throw);
 }
 
+ADD_TEST(ParseErrorOnInvalidToken) {
+	bool did_throw{};
+	try {
+		parse("`");
+	} catch (UnrecognizedToken const& e) {
+		EXPECT(e.token.location == 0);
+		EXPECT(e.token.lexeme == "`");
+		did_throw = true;
+	}
+	EXPECT(did_throw);
+
+	did_throw = false;
+	try {
+		parse("pi@");
+	} catch (UnrecognizedToken const& e) {
+		EXPECT(e.token.location == 2);
+		EXPECT(e.token.lexeme == "@");
+		did_throw = true;
+	}
+	EXPECT(did_throw);
+}
+
 ADD_TEST(ParsePrecedence) {
 	auto result = parse("1 + 2 * 3");
 	ASSERT(result != nullptr);