diff --git a/libavcodec/vp9.c b/libavcodec/vp9.c
index 3ad9056f18..6888326a05 100644
--- a/libavcodec/vp9.c
+++ b/libavcodec/vp9.c
@@ -425,7 +425,7 @@ static av_always_inline int inv_recenter_nonneg(int v, int m)
 // differential forward probability updates
 static int update_prob(VP56RangeCoder *c, int p)
 {
-    static const int inv_map_table[254] = {
+    static const int inv_map_table[255] = {
           7,  20,  33,  46,  59,  72,  85,  98, 111, 124, 137, 150, 163, 176,
         189, 202, 215, 228, 241, 254,   1,   2,   3,   4,   5,   6,   8,   9,
          10,  11,  12,  13,  14,  15,  16,  17,  18,  19,  21,  22,  23,  24,
@@ -444,7 +444,7 @@ static int update_prob(VP56RangeCoder *c, int p)
         207, 208, 209, 210, 211, 212, 213, 214, 216, 217, 218, 219, 220, 221,
         222, 223, 224, 225, 226, 227, 229, 230, 231, 232, 233, 234, 235, 236,
         237, 238, 239, 240, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251,
-        252, 253,
+        252, 253, 253,
     };
     int d;
 
@@ -474,6 +474,7 @@ static int update_prob(VP56RangeCoder *c, int p)
         if (d >= 65)
             d = (d << 1) - 65 + vp8_rac_get(c);
         d += 64;
+        av_assert2(d < FF_ARRAY_ELEMS(inv_map_table));
     }
 
     return p <= 128 ? 1 + inv_recenter_nonneg(inv_map_table[d], p - 1) :