diff --git a/sys/rump/librump/rumpvfs/rumpfs.c b/sys/rump/librump/rumpvfs/rumpfs.c
index ee3a40c71678..4d01e498dce6 100644
--- a/sys/rump/librump/rumpvfs/rumpfs.c
+++ b/sys/rump/librump/rumpvfs/rumpfs.c
@@ -1492,7 +1492,10 @@ rump_vop_write(void *v)
 			return ENOSPC;
 		rn->rn_dlen = newlen;
 		memset(rn->rn_data, 0, newlen);
-		memcpy(rn->rn_data, olddata, oldlen);
+		if (oldlen > 0) {
+			KASSERT(olddata == NULL);
+			memcpy(rn->rn_data, olddata, oldlen);
+		}
 		allocd = true;
 		uvm_vnp_setsize(vp, newlen);
 	}