Index: zlib-1.2.11/deflate.c
===================================================================
--- zlib-1.2.11.orig/deflate.c
+++ zlib-1.2.11/deflate.c
@@ -504,7 +504,7 @@ int ZEXPORT deflateResetKeep (strm)
         s->wrap == 2 ? crc32(0L, Z_NULL, 0) :
 #endif
         adler32(0L, Z_NULL, 0);
-    s->last_flush = Z_NO_FLUSH;
+    s->last_flush = -2;
 
     _tr_init(s);
 
@@ -601,13 +601,13 @@ int ZEXPORT deflateParams(strm, level, s
     func = configuration_table[s->level].func;
 
     if ((strategy != s->strategy || func != configuration_table[level].func ||
-         hook_flush != Z_NO_FLUSH) && s->high_water) {
+         hook_flush != Z_NO_FLUSH) && s->last_flush != -2) {
         /* Flush the last buffer: */
         int err = deflate(strm, RANK(hook_flush) > RANK(Z_BLOCK) ?
                           hook_flush : Z_BLOCK);
         if (err == Z_STREAM_ERROR)
             return err;
-        if (strm->avail_out == 0)
+        if (strm->avail_in || (s->strstart - s->block_start) + s->lookahead)
             return Z_BUF_ERROR;
     }
     if (s->level != level) {
Index: zlib-1.2.11/zlib.h
===================================================================
--- zlib-1.2.11.orig/zlib.h
+++ zlib-1.2.11/zlib.h
@@ -712,11 +712,12 @@ ZEXTERN int ZEXPORT deflateParams OF((z_
    used to switch between compression and straight copy of the input data, or
    to switch to a different kind of input data requiring a different strategy.
    If the compression approach (which is a function of the level) or the
-   strategy is changed, and if any input has been consumed in a previous
-   deflate() call, then the input available so far is compressed with the old
-   level and strategy using deflate(strm, Z_BLOCK).  There are three approaches
-   for the compression levels 0, 1..3, and 4..9 respectively.  The new level
-   and strategy will take effect at the next call of deflate().
+   strategy is changed, and if there have been any deflate() calls since the
+   state was initialized or reset, then the input available so far is
+   compressed with the old level and strategy using deflate(strm, Z_BLOCK).
+   There are three approaches for the compression levels 0, 1..3, and 4..9
+   respectively.  The new level and strategy will take effect at the next call
+   of deflate().
 
      If a deflate(strm, Z_BLOCK) is performed by deflateParams(), and it does
    not have enough output space to complete, then the parameter change will not