diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index f13ef4d..a3c9fd4 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -1265,6 +1265,10 @@ zstream_sync(struct zstream *z, Bytef *src, rb_len_t len)
     /* VALUE rest; */
     int err;
 
+    if (z->flags & ZSTREAM_IN_PROGRESS) {
+	rb_raise(cInProgressError, "zlib stream is in progress");
+    }
+
     if (!NIL_P(z->input)) {
 	z->stream.next_in = (Bytef*)RSTRING_PTR(z->input);
 	z->stream.avail_in = MAX_UINT(RSTRING_LEN(z->input));
@@ -2341,8 +2345,11 @@ rb_inflate_addstr(VALUE obj, VALUE src)
  *
  * Inputs +string+ into the end of input buffer and skips data until a full
  * flush point can be found.  If the point is found in the buffer, this method
- * flushes the buffer and returns false.  Otherwise it returns +true+ and the
- * following data of full flush point is preserved in the buffer.
+ * returns +true+ and the following data of full flush point is preserved in
+ * the buffer.  Otherwise it flushes the buffer and returns +false+.
+ *
+ * Raises a Zlib::InProgressError exception if called while the stream is
+ * being inflated.
  */
 static VALUE
 rb_inflate_sync(VALUE obj, VALUE src)
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index e1ecbf8..803642c 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -647,6 +647,19 @@ def test_sync
       assert_equal(true, z.sync("\0" * 100 + s))
     end
 
+    def test_sync_in_progress
+      src = "x" * 100_000
+      z = Zlib::Inflate.new
+      inflated = "".dup
+      z.inflate(Zlib.deflate(src)) do |chunk|
+        if inflated.empty?
+          assert_raise(Zlib::InProgressError) { z.sync("\0\0\xff\xff".b) }
+        end
+        inflated << chunk
+      end
+      assert_equal src, inflated
+    end
+
     def test_set_dictionary
       z = Zlib::Inflate.new
       assert_raise(Zlib::StreamError) { z.set_dictionary("foo") }