diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index bcdd32a..6d85f6e 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -1531,8 +1531,12 @@ static VALUE
 rb_zstream_set_avail_out(VALUE obj, VALUE size)
 {
     struct zstream *z = get_zstream(obj);
+    int n = NUM2INT(size);
 
-    zstream_expand_buffer_into(z, FIX2INT(size));
+    if (n < 0) {
+	rb_raise(rb_eArgError, "negative size");
+    }
+    zstream_expand_buffer_into(z, n);
     return size;
 }
 
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index e1ecbf8..ac4bdde 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -132,6 +132,14 @@ def test_avail
       assert_equal("foobar", Zlib::Inflate.inflate(s))
     end
 
+    def test_negative_avail_out
+      z = Zlib::Inflate.new
+      assert_raise(ArgumentError) { z.avail_out = -1 }
+      assert_equal("foo" * 100, z.inflate(Zlib::Deflate.deflate("foo" * 100)))
+    ensure
+      z&.close
+    end
+
     def test_expand_buffer;
       z = Zlib::Deflate.new
       src = "baz" * 1000