diff --git a/tests/test_CacheWriter.py b/tests/test_CacheWriter.py
index b9330757e412328ce149382f35c15dd38a21e4d3..1ceadce2cfe17eaadd3af6a004282dab688312a7 100644
--- a/tests/test_CacheWriter.py
+++ b/tests/test_CacheWriter.py
@@ -1,10 +1,10 @@
 import datetime
-import io
+import json
 import pathlib
 
 import pytest
 
-from . import any_date, relpath
+from . import any_date, relpath, StringIO
 import oxrlib.cache
 import oxrlib.errors
 
@@ -17,9 +17,19 @@ class TestCacheFile(oxrlib.cache.WriteCacheFile):
         assert args[0].startswith('w')
 
     def open(self):
-        open_file = io.StringIO()
-        open_file.name = self.path.as_posix()
-        return open_file
+        last_file = StringIO()
+        last_file.name = self.path.as_posix()
+        type(self).last_file = last_file
+        return self.last_file
+
+
+class FakeRate:
+    def __init__(self, date, base):
+        self.timestamp = datetime.datetime(date.year, date.month, date.day)
+        self.base = base
+
+    def serialize(self):
+        return "test data for {} in {}".format(self.base, self.timestamp.date())
 
 
 @pytest.fixture
@@ -33,3 +43,8 @@ def test_cache_success(dummycache, any_date):
     expect_name = CACHE_PATH / HISTORICAL_PATTERN.format(date=any_date.isoformat(), base=base)
     with dummycache.historical(any_date, base) as cache_file:
         assert pathlib.Path(cache_file.name) == expect_name
+
+def test_save_rate(dummycache, any_date):
+    rate = FakeRate(any_date, 'USD')
+    dummycache.save_rate(rate)
+    assert TestCacheFile.last_file.last_value == json.dumps(rate.serialize())