diff --git a/lexer.go b/lexer.go
index 817900c..9f214d0 100644
--- a/lexer.go
+++ b/lexer.go
@@ -386,7 +386,7 @@ func (lexer *Lexer) consumeUnquotedIdentifier() token {
 	start := lexer.currentPos - lexer.lastWidth
 	for {
 		r := lexer.next()
-		if r < 0 || r > 128 || identifierTrailingBits[uint64(r)/64]&(1<<(uint64(r)%64)) == 0 {
+		if r < 0 || r >= 128 || identifierTrailingBits[uint64(r)/64]&(1<<(uint64(r)%64)) == 0 {
 			lexer.back()
 			break
 		}
diff --git a/lexer_test.go b/lexer_test.go
index 307f2af..2baa30e 100644
--- a/lexer_test.go
+++ b/lexer_test.go
@@ -96,6 +96,10 @@ var lexingErrorTests = []struct {
 }{
 	{"'foo", "Missing closing single quote"},
 	{"[?foo==bar?]", "Unknown char '?'"},
+	// U+0080 is the first rune outside the ASCII bit mask used for
+	// unquoted identifiers. It must be rejected rather than indexed.
+	{"a\u0080", "Unknown char '\\u0080'"},
+	{"a\u0080b", "Unknown char '\\u0080'"},
 }
 
 func TestLexingErrors(t *testing.T) {