diff --git a/config.py b/config.py
index 9edb6a6..2482292 100644
--- a/config.py
+++ b/config.py
@@ -414,7 +414,7 @@ YOUTUBE_API_KEY = os.getenv('YOUTUBE_API_KEY', '')
 # ==========================================
 # 系統版本與路徑
 # ==========================================
-SYSTEM_VERSION = "V10.802"
+SYSTEM_VERSION = "V10.803"
 LOG_FILE_PATH = os.path.join(BASE_DIR, 'logs/system.log')
 public_url = PUBLIC_URL  # 用於模板顯示
 
diff --git a/routes/sales_routes.py b/routes/sales_routes.py
index 62ecc5e..ecb52e5 100644
--- a/routes/sales_routes.py
+++ b/routes/sales_routes.py
@@ -520,6 +520,16 @@ def _filter_growth_payload(chart_data, kpi, start_month=None, end_month=None):
     return filtered, filtered_kpi, analysis_period, True
 
 
+def _sorted_valid_month_labels(values):
+    """Normalize mixed pandas month values before chart-axis sorting."""
+    labels = set()
+    for value in values:
+        parsed = parse_month(value)
+        if parsed:
+            labels.add(parsed.strftime('%Y-%m'))
+    return sorted(labels)
+
+
 def _build_growth_kpi(kpi_row):
     if not kpi_row:
         return None
@@ -1889,7 +1899,7 @@ def sales_analysis():
             # Y軸: 分類 (使用 top_cats_season 的索引)
             
             # 取得所有月份並排序
-            all_months_sorted = sorted(target_df['_month_str'].unique())
+            all_months_sorted = _sorted_valid_month_labels(target_df['_month_str'].unique())
             month_map = {m: i for i, m in enumerate(all_months_sorted)}
             cat_map = {c: i for i, c in enumerate(top_cats_season)}
             
diff --git a/tests/test_analysis_period_linkage.py b/tests/test_analysis_period_linkage.py
index 8bf588a..8e916fa 100644
--- a/tests/test_analysis_period_linkage.py
+++ b/tests/test_analysis_period_linkage.py
@@ -14,7 +14,7 @@ from routes.monthly_routes import (
     _resolve_monthly_period,
     get_monthly_summary_data,
 )
-from routes.sales_routes import _filter_growth_payload
+from routes.sales_routes import _filter_growth_payload, _sorted_valid_month_labels
 from services.analysis_period_service import make_analysis_period, make_month_period
 
 
@@ -250,6 +250,17 @@ def test_growth_period_slices_every_series_and_recomputes_period_kpis():
     assert period["end_month"] == "2026-05"
 
 
+def test_sales_seasonality_month_axis_ignores_nan_and_invalid_values():
+    assert _sorted_valid_month_labels([
+        "2026-04",
+        float("nan"),
+        None,
+        "invalid",
+        "2025-12",
+        "2026-04-30",
+    ]) == ["2025-12", "2026-04"]
+
+
 def test_monthly_frontend_has_replayable_period_and_no_fixed_years():
     script = (ROOT / "web/static/js/page-monthly-summary.js").read_text(encoding="utf-8")
     route = (ROOT / "routes/monthly_routes.py").read_text(encoding="utf-8")