diff --git a/av1/encoder/cost.h b/av1/encoder/cost.h
index af5b098..be0241a 100644
--- a/av1/encoder/cost.h
+++ b/av1/encoder/cost.h
@@ -30,6 +30,10 @@
 
 // Calculate the cost of a symbol with probability p15 / 2^15
 static INLINE int av1_cost_symbol(aom_cdf_prob p15) {
+  // p15 can be out of range [1, CDF_PROB_TOP - 1]. Clamping it, so that the
+  // following cost calculation works correctly. Otherwise, if p15 =
+  // CDF_PROB_TOP, shift would be -1, and "p15 << shift" would be wrong.
+  p15 = (aom_cdf_prob)clamp(p15, 1, CDF_PROB_TOP - 1);
   assert(0 < p15 && p15 < CDF_PROB_TOP);
   const int shift = CDF_PROB_BITS - 1 - get_msb(p15);
   const int prob = get_prob(p15 << shift, CDF_PROB_TOP);