diff --git a/inc/qcbor/UsefulBuf.h b/inc/qcbor/UsefulBuf.h
index e7f4d4f..2b85833 100644
--- a/inc/qcbor/UsefulBuf.h
+++ b/inc/qcbor/UsefulBuf.h
@@ -2185,7 +2185,7 @@
    UsefulBuf R;
 
    R.len = UsefulOutBuf_RoomLeft(pUOutBuf);
-   if(R.len > 0) {
+   if(R.len > 0 && pUOutBuf->UB.ptr != NULL) {
       R.ptr = (uint8_t *)pUOutBuf->UB.ptr + pUOutBuf->data_len;
    } else {
       R.ptr = NULL;