diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 82345cd49d..2a5cee17ed 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -497,6 +497,10 @@ static void ac3_decode_transform_coeffs_ch(AC3DecodeContext *s, int ch_index, ma
             break;
         default: /* 6 to 15 */
             /* Shift mantissa and sign-extend it. */
+            if (bap > 15) {
+                av_log(s->avctx, AV_LOG_ERROR, "bap %d is invalid in plain AC-3\n", bap);
+                bap = 15;
+            }
             mantissa = get_sbits(gbc, quantization_tab[bap]);
             mantissa <<= 24 - quantization_tab[bap];
             break;