diff --git a/deflate.c b/deflate.c
index 8a1a91e..f146001 100644
--- a/deflate.c
+++ b/deflate.c
@@ -586,7 +586,8 @@
     }
     func = configuration_table[s->level].func;
 
-    if ((strategy != s->strategy || func != configuration_table[level].func)) {
+    if ((strategy != s->strategy || func != configuration_table[level].func) &&
+        s->high_water) {
         /* Flush the last buffer: */
         int err = deflate(strm, Z_BLOCK);
         if (err == Z_STREAM_ERROR)
diff --git a/zlib.h b/zlib.h
index feabc54..a4b3baf 100644
--- a/zlib.h
+++ b/zlib.h
@@ -712,10 +712,11 @@
    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, 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 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().
 
      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