diff --git a/libavcodec/g723_1.c b/libavcodec/g723_1.c
index 4904e52ac0..80c7d1f491 100644
--- a/libavcodec/g723_1.c
+++ b/libavcodec/g723_1.c
@@ -386,7 +386,7 @@ static void lsp2lpc(int16_t *lpc)
 
     /* Calculate negative cosine */
     for (j = 0; j < LPC_ORDER; j++) {
-        int index     = lpc[j] >> 7;
+        int index     = (lpc[j] >> 7) & 0x1FF;
         int offset    = lpc[j] & 0x7f;
         int temp1     = cos_tab[index] << 16;
         int temp2     = (cos_tab[index + 1] - cos_tab[index]) *
diff --git a/libavcodec/g723_1_data.h b/libavcodec/g723_1_data.h
index 04f8a06e37..c4c4d77a62 100644
--- a/libavcodec/g723_1_data.h
+++ b/libavcodec/g723_1_data.h
@@ -56,7 +56,7 @@ static const int16_t dc_lsp[LPC_ORDER] = {
 };
 
 /* Cosine table scaled by 2^14 */
-static const int16_t cos_tab[COS_TBL_SIZE] = {
+static const int16_t cos_tab[COS_TBL_SIZE + 1] = {
     16384,  16383,  16379,  16373,  16364,  16353,  16340,  16324,
     16305,  16284,  16261,  16235,  16207,  16176,  16143,  16107,
     16069,  16029,  15986,  15941,  15893,  15843,  15791,  15736,
@@ -121,6 +121,7 @@ static const int16_t cos_tab[COS_TBL_SIZE] = {
     15679,  15736,  15791,  15843,  15893,  15941,  15986,  16029,
     16069,  16107,  16143,  16176,  16207,  16235,  16261,  16284,
     16305,  16324,  16340,  16353,  16364,  16373,  16379,  16383,
+    16384
 };
 
 /* LSP VQ tables */