diff --git a/crc32.c b/crc32.c
index 05a0d81..027b033 100644
--- a/crc32.c
+++ b/crc32.c
@@ -1030,6 +1030,8 @@ unsigned long ZEXPORT crc32(unsigned long crc, const unsigned char FAR *buf,
 
 /* ========================================================================= */
 uLong ZEXPORT crc32_combine64(uLong crc1, uLong crc2, z_off64_t len2) {
+    if (len2 < 0)
+        return 0;
 #ifdef DYNAMIC_CRC_TABLE
     once(&made, make_crc_table);
 #endif /* DYNAMIC_CRC_TABLE */
@@ -1043,6 +1045,8 @@ uLong ZEXPORT crc32_combine(uLong crc1, uLong crc2, z_off_t len2) {
 
 /* ========================================================================= */
 uLong ZEXPORT crc32_combine_gen64(z_off64_t len2) {
+    if (len2 < 0)
+        return 0;
 #ifdef DYNAMIC_CRC_TABLE
     once(&made, make_crc_table);
 #endif /* DYNAMIC_CRC_TABLE */
diff --git a/zlib.h b/zlib.h
index 8d4b932..8c7f8ac 100644
--- a/zlib.h
+++ b/zlib.h
@@ -1758,14 +1758,14 @@ ZEXTERN uLong ZEXPORT crc32_combine(uLong crc1, uLong crc2, z_off_t len2);
    seq1 and seq2 with lengths len1 and len2, CRC-32 check values were
    calculated for each, crc1 and crc2.  crc32_combine() returns the CRC-32
    check value of seq1 and seq2 concatenated, requiring only crc1, crc2, and
-   len2. len2 must be non-negative.
+   len2. len2 must be non-negative, otherwise zero is returned.
 */
 
 /*
 ZEXTERN uLong ZEXPORT crc32_combine_gen(z_off_t len2);
 
      Return the operator corresponding to length len2, to be used with
-   crc32_combine_op(). len2 must be non-negative.
+   crc32_combine_op(). len2 must be non-negative, otherwise zero is returned.
 */
 
 ZEXTERN uLong ZEXPORT crc32_combine_op(uLong crc1, uLong crc2, uLong op);