diff --git a/fmt/fmt_asn1derlength.c b/fmt/fmt_asn1derlength.c
index f4814d7..70da5e2 100644
--- a/fmt/fmt_asn1derlength.c
+++ b/fmt/fmt_asn1derlength.c
@@ -1,4 +1,5 @@
 #include <libowfat/fmt.h>
+#include <assert.h>
 
 static size_t asn1derlengthbytesneeded(unsigned long long l) {
   size_t needed;
@@ -15,9 +16,7 @@ size_t fmt_asn1derlength(char* dest,unsigned long long l) {
     if (dest) *dest=l&0x7f;
     return 1;
   }
-  for (i=1; i<sizeof l; ++i)
-    if (!(l>>(i*8)))
-      break;
+  i=asn1derlengthbytesneeded(l);
   if (dest) {
     size_t j=i;
     *dest=(char)(0x80+i); ++dest;