diff --git a/flask/testing.py b/flask/testing.py
index c1844c00..dfdaff57 100644
--- a/flask/testing.py
+++ b/flask/testing.py
@@ -108,6 +108,8 @@ class FlaskClient(Client):
             self.context_preserved = _request_ctx_stack.top is not old
 
     def __enter__(self):
+        if self.preserve_context:
+            raise RuntimeError('Cannot nest client invocations')
         self.preserve_context = True
         return self