diff --git a/library/bignum.c b/library/bignum.c
index 09ce030..1b7ff58 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -701,7 +701,7 @@
     }
 
     *p++ = '\0';
-    *olen = p - buf;
+    *olen = (size_t) (p - buf);
 
 cleanup: