diff --git a/lib/asn1/der_locl.h b/lib/asn1/der_locl.h
index 4b67803da..578a0707c 100644
--- a/lib/asn1/der_locl.h
+++ b/lib/asn1/der_locl.h
@@ -54,7 +54,7 @@
 #include <der.h>
 
 #ifndef HAVE_TIMEGM
-time_t timegm (struct tm *);
+time_t _der_timegm (struct tm *);
 #endif
 
 size_t _heim_len_unsigned (unsigned);