diff --git a/src/core/def.c b/src/core/def.c
index 89fc8ae7..9da36fee 100644
--- a/src/core/def.c
+++ b/src/core/def.c
@@ -235,6 +235,6 @@ lwip_itoa(char *result, size_t bufsize, int number)
     return;
   }
   /* move from temporary buffer to output buffer (sign is not moved) */
-  memmove(res, tmp, (result + bufsize) - tmp);
+  memmove(res, tmp, (size_t)((result + bufsize) - tmp));
 }
 #endif