diff --git a/ast/ast.go b/ast/ast.go
index f3efbfbf..f78e6d30 100644
--- a/ast/ast.go
+++ b/ast/ast.go
@@ -243,7 +243,7 @@ func Integer(tk *token.Token) Node {
 		negativePrefix := ""
 		if value[0] == '-' {
 			skipCharacterNum++
-			if value[2] == 'o' {
+			if len(value) > 2 && value[2] == 'o' {
 				skipCharacterNum++
 			}
 			negativePrefix = "-"