diff -Nur zstd-1.5.0/lib/compress/zstd_compress_internal.h new-zstd/lib/compress/zstd_compress_internal.h
--- zstd-1.5.0/lib/compress/zstd_compress_internal.h	2021-05-14 22:59:34.000000000 +0800
+++ new-zstd/lib/compress/zstd_compress_internal.h	2021-11-22 20:00:33.612480981 +0800
@@ -199,6 +199,8 @@
                                 */
 } ZSTD_window_t;
 
+#define ZSTD_WINDOW_START_INDEX 2
+
 typedef struct ZSTD_matchState_t ZSTD_matchState_t;
 
 #define ZSTD_ROW_HASH_CACHE_SIZE 8       /* Size of prefetching hash cache for row-based matchfinder */
@@ -884,9 +886,9 @@
 
 MEM_STATIC U32 ZSTD_window_isEmpty(ZSTD_window_t const window)
 {
-    return window.dictLimit == 1 &&
-           window.lowLimit == 1 &&
-           (window.nextSrc - window.base) == 1;
+    return window.dictLimit == ZSTD_WINDOW_START_INDEX &&
+           window.lowLimit == ZSTD_WINDOW_START_INDEX &&
+           (window.nextSrc - window.base) == ZSTD_WINDOW_START_INDEX;
 }
 
 /**
@@ -1151,9 +1153,10 @@
     ZSTD_memset(window, 0, sizeof(*window));
     window->base = (BYTE const*)"";
     window->dictBase = (BYTE const*)"";
-    window->dictLimit = 1;    /* start from 1, so that 1st position is valid */
-    window->lowLimit = 1;     /* it ensures first and later CCtx usages compress the same */
-    window->nextSrc = window->base + 1;   /* see issue #1241 */
+    ZSTD_STATIC_ASSERT(ZSTD_DUBT_UNSORTED_MARK < ZSTD_WINDOW_START_INDEX);
+    window->dictLimit = ZSTD_WINDOW_START_INDEX;    /* start from >0, so that 1st position is valid */
+    window->lowLimit = ZSTD_WINDOW_START_INDEX;     /* it ensures first and later CCtx usages compress the same */
+    window->nextSrc = window->base + ZSTD_WINDOW_START_INDEX;   /* see issue #1241 */
     window->nbOverflowCorrections = 0;
 }