diff --git a/src/rds.c b/src/rds.c
index 45bea9b..cfe72d3 100644
--- a/src/rds.c
+++ b/src/rds.c
@@ -57,7 +57,7 @@ void get_rds_sequence_group(RDSEncoder* enc, RDSGroup *group, uint8_t good_group
 	}
 }
 
-uint8_t check_rds_good_group(RDSEncoder* enc, char* grp) {
+uint8_t check_rds_good_group(RDSEncoder* enc, const char* grp) {
 	if(*grp == 2) {
 		if(enc->data[enc->program].ecc != 0 || enc->data[enc->program].slc_data != 0) return 1;
 		return 0;
diff --git a/src/rds.h b/src/rds.h
index 52fce22..48df471 100644
--- a/src/rds.h
+++ b/src/rds.h
@@ -158,7 +158,7 @@ void reset_rds_state(RDSEncoder* enc, uint8_t program);
 void set_rds_defaults(RDSEncoder* enc, uint8_t program);
 void init_rds_encoder(RDSEncoder* enc);
 void add_checkwords(RDSGroup *group, uint8_t *bits);
-uint8_t check_rds_good_group(RDSEncoder* enc, char* grp);
+uint8_t check_rds_good_group(RDSEncoder* enc, const char* grp);
 void get_rds_sequence_group(RDSEncoder* enc, RDSGroup *group, uint8_t good_group, char* grp);
 void get_rds_group(RDSEncoder* enc, RDSGroup *group, uint8_t stream);
 void get_rds_bits(RDSEncoder* enc, uint8_t *bits, uint8_t stream);