diff --git a/frontend/app/history/page.tsx b/frontend/app/history/page.tsx
index 63b633f..073835e 100644
--- a/frontend/app/history/page.tsx
+++ b/frontend/app/history/page.tsx
@@ -51,7 +51,7 @@ export default function HistoryPage() {
               <LineChart data={chartData} margin={{ top: 4, right: 8, bottom: 4, left: 8 }}>
                 <XAxis dataKey="time" tick={{ fill: "#64748b", fontSize: 10 }} tickLine={false} interval="preserveStartEnd" />
                 <YAxis tick={{ fill: "#64748b", fontSize: 10 }} tickLine={false} axisLine={false} tickFormatter={(v) => `${v.toFixed(3)}%`} width={60} />
-                <Tooltip formatter={(v: number) => `${v.toFixed(4)}%`} contentStyle={{ background: "#1e293b", border: "1px solid #475569", borderRadius: 8 }} />
+                <Tooltip formatter={(v: unknown) => typeof v === "number" ? `${v.toFixed(4)}%` : v} contentStyle={{ background: "#1e293b", border: "1px solid #475569", borderRadius: 8 }} />
                 <Legend wrapperStyle={{ fontSize: 12, color: "#94a3b8" }} />
                 <ReferenceLine y={0} stroke="#475569" strokeDasharray="4 2" />
                 <Line type="monotone" dataKey="BTC" stroke="#06b6d4" strokeWidth={1.5} dot={false} connectNulls />