diff --git a/libfaketime/__init__.py b/libfaketime/__init__.py
index e256b2a..422b406 100644
--- a/libfaketime/__init__.py
+++ b/libfaketime/__init__.py
@@ -237,7 +237,7 @@ def __exit__(self, *exc):
                     del os.environ["FAKETIME"]
 
             if self._prev_fmt is not None:
-                os.environ["FAKETIME_FMT"] = self._prev_spec
+                os.environ["FAKETIME_FMT"] = self._prev_fmt
             else:
                 del os.environ["FAKETIME_FMT"]
 
diff --git a/test/test_faketime.py b/test/test_faketime.py
index afde4e7..a2f2599 100644
--- a/test/test_faketime.py
+++ b/test/test_faketime.py
@@ -83,6 +83,18 @@ def test_nested_fake_time(self):
 
         self._assert_time_not_faked()
 
+    def test_nested_fake_time_restores_format(self):
+        """Leaving a nested context restores FAKETIME_FMT, and not FAKETIME."""
+        with fake_time("2000-01-01 10:00:05"):
+            outer_fmt = os.environ["FAKETIME_FMT"]
+
+            with fake_time("2001-01-01 10:00:05"):
+                pass
+
+            assert os.environ["FAKETIME_FMT"] == outer_fmt
+
+        assert "FAKETIME_FMT" not in os.environ
+
     def test_freeze_time_alias(self):
         with freeze_time("2000-01-01 10:00:05"):
             assert datetime.datetime(2000, 1, 1, 10, 0, 5) == datetime.datetime.now()