diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index f13ef4d..966c280 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -4338,7 +4338,7 @@ gzreader_skip_linebreaks(struct gzfile *gz)
 static void
 rscheck(const char *rsptr, rb_len_t rslen, VALUE rs)
 {
-    if (RSTRING_PTR(rs) != rsptr && RSTRING_LEN(rs) != rslen)
+    if (RSTRING_PTR(rs) != rsptr || RSTRING_LEN(rs) != rslen)
 	rb_raise(rb_eRuntimeError, "rs modified");
 }
 
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index e1ecbf8..2a985ec 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -987,6 +987,19 @@ def test_read0
       assert_equal("", Zlib::GzipReader.new(StringIO.new(D0)).read(0))
     end
 
+    def test_gets_separator_modified_by_io
+      rs = "-" * 1000
+      src = StringIO.new(Zlib.gzip(Random.new(0).bytes(200_000).tr("-", "x") + rs))
+      n = 0
+      io = Object.new
+      io.define_singleton_method(:readpartial) do |len, buf = nil|
+        (rs.clear << "-" * 1000) if (n += 1) == 300
+        src.readpartial([len, 100].min, *buf)
+      end
+      gz = Zlib::GzipReader.new(io)
+      assert_raise_with_message(RuntimeError, "rs modified") { gz.gets(rs) }
+    end
+
     def test_ungetc
       s = "".dup
       w = Zlib::GzipWriter.new(StringIO.new(s))