diff --git a/tests/io/write_ext.py b/tests/io/write_ext.py
new file mode 100644
index 0000000000..f4ed0d3685
--- /dev/null
+++ b/tests/io/write_ext.py
@@ -0,0 +1,25 @@
+import uio
+
+try:
+    uio.BytesIO
+except AttributeError:
+    import sys
+    print('SKIP')
+    sys.exit()
+
+buf = uio.BytesIO()
+
+buf.write(b"foo", 2)
+print(buf.getvalue())
+
+buf.write(b"foo", 100)
+print(buf.getvalue())
+
+buf.write(b"foobar", 1, 3)
+print(buf.getvalue())
+
+buf.write(b"foobar", 1, 100)
+print(buf.getvalue())
+
+buf.write(b"foobar", 100, 100)
+print(buf.getvalue())
diff --git a/tests/io/write_ext.py.exp b/tests/io/write_ext.py.exp
new file mode 100644
index 0000000000..0f9c6bfbc1
--- /dev/null
+++ b/tests/io/write_ext.py.exp
@@ -0,0 +1,5 @@
+b'fo'
+b'fofoo'
+b'fofoooob'
+b'fofooooboobar'
+b'fofooooboobar'