diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index f27359c..f429ba6 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -155,7 +155,7 @@
 
 uint32_t osmo_isqrt32(uint32_t x);
 
-const char osmo_luhn(const char* in, int in_len);
+char osmo_luhn(const char* in, int in_len);
 
 /*! State for OSMO_STRBUF_APPEND() and OSMO_STRBUF_PRINTF(). See there for examples. */
 struct osmo_strbuf {