diff --git a/internal/captcha/handler.go b/internal/captcha/handler.go
index eb7461b..46bf0e1 100644
--- a/internal/captcha/handler.go
+++ b/internal/captcha/handler.go
@@ -327,7 +327,11 @@ func (h *Handler) Math(imageBase64 string) (result string, err error) {
 			maxIdx = c
 		}
 	}
-	fmt.Printf("DEBUG: Position 0 argmax = %d (char: %s)\n", maxIdx, string("0123456789+-*/÷×=?."[maxIdx-1]))
+	// 【注意】这里用 string 索引访问多字节字符可能有问题，仅用于调试
+	debugChars := []string{"", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "+", "-", "*", "/", "÷", "×", "=", "?"}
+	if maxIdx >= 0 && maxIdx < len(debugChars) {
+		fmt.Printf("DEBUG: Position 0 argmax = %d (char: %s)\n", maxIdx, debugChars[maxIdx])
+	}
 
 	if len(output) == 0 {
 		return "", fmt.Errorf("模型输出为空")