diff --git a/lib/core/ogs-conv.c b/lib/core/ogs-conv.c
index d0ffc28e5..f4d3fa188 100644
--- a/lib/core/ogs-conv.c
+++ b/lib/core/ogs-conv.c
@@ -52,7 +52,7 @@ int ogs_ascii_to_hex(char *in, int in_len, void *out, int out_len)
     return j;
 }
 
-void *ogs_hex_to_ascii(void *in, int in_len, void *out, int out_len)
+void *ogs_hex_to_ascii(const void *in, int in_len, void *out, int out_len)
 {
     char *p, *last;
     int i = 0, l, off = 0;
diff --git a/lib/core/ogs-conv.h b/lib/core/ogs-conv.h
index df092e40c..0374aaf82 100644
--- a/lib/core/ogs-conv.h
+++ b/lib/core/ogs-conv.h
@@ -36,7 +36,7 @@ static ogs_inline void *ogs_hex_from_string(
     return out;
 }
 
-void *ogs_hex_to_ascii(void *in, int in_len, void *out, int out_len);
+void *ogs_hex_to_ascii(const void *in, int in_len, void *out, int out_len);
 void *ogs_uint64_to_buffer(uint64_t num, int size, void *buffer);
 uint64_t ogs_buffer_to_uint64(void *buffer, int size);
 void *ogs_bcd_to_buffer(const char *in, void *out, int *out_len);