--- a/taskcluster/taskgraph/util/seta.py
+++ b/taskcluster/taskgraph/util/seta.py
@@ -59,16 +59,20 @@ class SETA(object):
         except exceptions.HTTPError:
             logger.warning("We got bad Http response from ouija,"
                            " we will treat all test tasks as high value.")
 
         # We just print the error out as a debug message if we failed to catch the exception above
         except exceptions.RequestException as error:
             logger.warning(error)
 
+        # When we get invalid JSON (i.e. 500 error), it results in a ValueError (bug 1313426)
+        except ValueError as error:
+            logger.warning("Invalid JSON, possible server error: {}".format(error))
+
         return low_value_tasks
 
     def is_low_value_task(self, label, project):
         # cache the low value tasks per project to avoid repeated SETA server queries
         if project not in self.low_value_tasks:
             self.low_value_tasks[project] = self.query_low_value_tasks(project)
         return label in self.low_value_tasks[project]