diff --git a/include/osmocom/sgsn/crc24.h b/include/osmocom/sgsn/crc24.h
index 756638c..c913eaf 100644
--- a/include/osmocom/sgsn/crc24.h
+++ b/include/osmocom/sgsn/crc24.h
@@ -5,6 +5,6 @@
 
 #define INIT_CRC24	0xffffff
 
-uint32_t crc24_calc(uint32_t fcs, uint8_t *cp, unsigned int len);
+uint32_t crc24_calc(uint32_t fcs, const uint8_t *cp, unsigned int len);
 
 #endif