diff --git a/lib/iris/tests/test_image_json.py b/lib/iris/tests/test_image_json.py
index d767245a84..ae144ce3f6 100644
--- a/lib/iris/tests/test_image_json.py
+++ b/lib/iris/tests/test_image_json.py
@@ -35,7 +35,7 @@
 from threading import Thread
 
 # maximum number of threads for multi-threading code
-MAXTHREADS = 128
+MAXTHREADS = 8
 
 # Turn down requests logging
 logging.getLogger("requests").setLevel(logging.CRITICAL)
@@ -58,8 +58,7 @@ def run(self):
             resource = self.queue.get()
             try:
                 result = requests.head(resource)
-                if (result.status_code == 200 and
-                   resource.startswith('https://scitools.github.io')):
+                if result.status_code == 200:
                     self.deque.append(resource)
                 else:
                     msg = '{} is not resolving correctly.'.format(resource)
@@ -80,7 +79,11 @@ def test_resolve(self):
         exceptions = deque()
         uri_queue = Queue()
         for uri in uris:
-            uri_queue.put(uri)
+            if uri.startswith('https://scitools.github.io'):
+                uri_queue.put(uri)
+            else:
+                msg = '{} is not a valid resource.'.format(uri)
+                exceptions.append(ValueError(msg))
 
         for i in range(MAXTHREADS):
             _ResolveWorkerThread(uri_queue, uri_list, exceptions).start()