diff --git a/profiler.py b/profiler.py
index 3b7cd78..07cf2cf 100644
--- a/profiler.py
+++ b/profiler.py
@@ -325,6 +325,7 @@ class ProfilerWSGIMiddleware(object):
 
     def __init__(self, app):
         template.register_template_library('gae_mini_profiler.templatetags')
+        self._lock = threading.RLock()
         self.app = app
         self.app_clean = app
         self.prof = None
@@ -337,18 +338,24 @@ def __init__(self, app):
         self.end = None
 
     def __call__(self, environ, start_response):
-
-        # Start w/ a non-profiled app at the beginning of each request
-        self.app = self.app_clean
-        self.prof = None
-        self.recorder = None
-        self.temporary_redirect = False
-        self.simple_timing = cookies.get_cookie_value("g-m-p-disabled") == "1"
-        requeststore.clear_id()
-
         # Never profile calls to the profiler itself to avoid endless recursion.
-        if config.should_profile(environ) and not environ.get("PATH_INFO", "").startswith("/gae_mini_profiler/"):
+        if not config.should_profile(environ) or environ.get("PATH_INFO", "").startswith("/gae_mini_profiler/"):
+            result = self.app(environ, start_response)
+            for value in result:
+                yield value
+            return
 
+        try:
+            self._lock.acquire()
+    
+            # Start w/ a non-profiled app at the beginning of each request
+            self.app = self.app_clean
+            self.prof = None
+            self.recorder = None
+            self.temporary_redirect = False
+            self.simple_timing = cookies.get_cookie_value("g-m-p-disabled") == "1"
+            requeststore.clear_id()
+            
             # Set a random ID for this request so we can look up stats later
             request_id = requeststore.generate_id()
 
@@ -439,10 +446,8 @@ def wrapped_appstats_app(environ, start_response):
             self.prof = None
             requeststore.clear_id()
 
-        else:
-            result = self.app(environ, start_response)
-            for value in result:
-                yield value
+        finally:
+            self._lock.release()
 
     def add_handler(self):
         if self.handler is None: