diff --git a/libavutil/lzo.c b/libavutil/lzo.c
index e49b83e0a2..0c497a5cf3 100644
--- a/libavutil/lzo.c
+++ b/libavutil/lzo.c
@@ -88,6 +88,10 @@ static inline int get_len(LZOContext *c, int x, int mask) {
 static inline void copy(LZOContext *c, int cnt) {
     register const uint8_t *src = c->in;
     register uint8_t *dst = c->out;
+    if (cnt < 0) {
+        c->error |= AV_LZO_ERROR;
+        return;
+    }
     if (cnt > c->in_end - src) {
         cnt = FFMAX(c->in_end - src, 0);
         c->error |= AV_LZO_INPUT_DEPLETED;
@@ -113,13 +117,17 @@ static inline void memcpy_backptr(uint8_t *dst, int back, int cnt);
 /**
  * @brief Copies previously decoded bytes to current position.
  * @param back how many bytes back we start
- * @param cnt number of bytes to copy, must be >= 0
+ * @param cnt number of bytes to copy, must be > 0
  *
  * cnt > back is valid, this will copy the bytes we just copied,
  * thus creating a repeating pattern with a period length of back.
  */
 static inline void copy_backptr(LZOContext *c, int back, int cnt) {
     register uint8_t *dst = c->out;
+    if (cnt <= 0) {
+        c->error |= AV_LZO_ERROR;
+        return;
+    }
     if (dst - c->out_start < back) {
         c->error |= AV_LZO_INVALID_BACKPTR;
         return;