diff --git a/include/osmocom/coding/gsm0503_coding.h b/include/osmocom/coding/gsm0503_coding.h
index 13d5344..3c51127 100644
--- a/include/osmocom/coding/gsm0503_coding.h
+++ b/include/osmocom/coding/gsm0503_coding.h
@@ -56,7 +56,7 @@
 	int *n_errors, int *n_bits_total);
 
 int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
-	int codec_mode_req, uint8_t *codec, int codecs, uint8_t ft,
+	int codec_mode_req, const uint8_t *codec, int codecs, uint8_t ft,
 	uint8_t cmr);
 int gsm0503_tch_afs_decode(uint8_t *tch_data, const sbit_t *bursts,
 	int codec_mode_req, uint8_t *codec, int codecs, uint8_t *ft,
@@ -66,7 +66,7 @@
 	uint8_t *cmr, int *n_errors, int *n_bits_total, uint8_t *dtx);
 
 int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
-	int codec_mode_req, uint8_t *codec, int codecs, uint8_t ft, uint8_t cmr);
+	int codec_mode_req, const uint8_t *codec, int codecs, uint8_t ft, uint8_t cmr);
 int gsm0503_tch_ahs_decode(uint8_t *tch_data, const sbit_t *bursts, int odd,
 	int codec_mode_req, uint8_t *codec, int codecs, uint8_t *ft,
 	uint8_t *cmr, int *n_errors, int *n_bits_total);
diff --git a/src/coding/gsm0503_coding.c b/src/coding/gsm0503_coding.c
index f8840c9..8effef5 100644
--- a/src/coding/gsm0503_coding.c
+++ b/src/coding/gsm0503_coding.c
@@ -2447,7 +2447,7 @@
  *  \param[in] cmr Codec Mode Request (used in codec_mode_req = 1 only)
  *  \returns 0 in case of success; negative on error */
 int gsm0503_tch_afs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
-	int codec_mode_req, uint8_t *codec, int codecs, uint8_t ft,
+	int codec_mode_req, const uint8_t *codec, int codecs, uint8_t ft,
 	uint8_t cmr)
 {
 	ubit_t iB[912], cB[456], h;
@@ -2935,7 +2935,7 @@
  *  \param[in] cmr Codec Mode Request (used in codec_mode_req = 1 only)
  *  \returns 0 in case of success; negative on error */
 int gsm0503_tch_ahs_encode(ubit_t *bursts, const uint8_t *tch_data, int len,
-	int codec_mode_req, uint8_t *codec, int codecs, uint8_t ft,
+	int codec_mode_req, const uint8_t *codec, int codecs, uint8_t ft,
 	uint8_t cmr)
 {
 	ubit_t iB[912], cB[456], h;