diff --git a/lib/deprecated/zbuff_compress.c b/lib/deprecated/zbuff_compress.c
index 8adbaec26..f39c60d89 100644
--- a/lib/deprecated/zbuff_compress.c
+++ b/lib/deprecated/zbuff_compress.c
@@ -72,6 +72,7 @@ size_t ZBUFF_compressInit_advanced(ZBUFF_CCtx* zbc,
                                    const void* dict, size_t dictSize,
                                    ZSTD_parameters params, unsigned long long pledgedSrcSize)
 {
+    if (pledgedSrcSize==0) pledgedSrcSize = ZSTD_CONTENTSIZE_UNKNOWN;  /* preserve "0 == unknown" behavior */
     return ZSTD_initCStream_advanced(zbc, dict, dictSize, params, pledgedSrcSize);
 }
 
diff --git a/tests/zbufftest.c b/tests/zbufftest.c
index e64c886a5..ce5e5518a 100644
--- a/tests/zbufftest.c
+++ b/tests/zbufftest.c
@@ -357,7 +357,7 @@ static int fuzzerTests(U32 seed, U32 nbTests, unsigned startTest, double compres
             {   ZSTD_parameters params = ZSTD_getParams(cLevel, 0, dictSize);
                 params.fParams.checksumFlag = FUZ_rand(&lseed) & 1;
                 params.fParams.noDictIDFlag = FUZ_rand(&lseed) & 1;
-                {   size_t const initError = ZBUFF_compressInit_advanced(zc, dict, dictSize, params, 0);
+                {   size_t const initError = ZBUFF_compressInit_advanced(zc, dict, dictSize, params, ZSTD_CONTENTSIZE_UNKNOWN);
                     CHECK (ZBUFF_isError(initError),"init error : %s", ZBUFF_getErrorName(initError));
         }   }   }