diff --git a/tests/fixtures/config/multi-tenant-semaphore/git/tenant-one-config/zuul.yaml b/tests/fixtures/config/multi-tenant-semaphore/git/tenant-one-config/zuul.yaml
index ca936b9214..9f3c5f663e 100644
--- a/tests/fixtures/config/multi-tenant-semaphore/git/tenant-one-config/zuul.yaml
+++ b/tests/fixtures/config/multi-tenant-semaphore/git/tenant-one-config/zuul.yaml
@@ -1,6 +1,9 @@
 - job:
     name: project1-test1
     semaphore: test-semaphore
+    vars:
+      semaphore:
+        test: 42
     run: playbooks/project1-test1.yaml
 
 - project:
diff --git a/zuul/lib/jsonutil.py b/zuul/lib/jsonutil.py
index d9f4eeb7a4..02f6f6da93 100644
--- a/zuul/lib/jsonutil.py
+++ b/zuul/lib/jsonutil.py
@@ -17,7 +17,11 @@ import types
 class ZuulJSONEncoder(json.JSONEncoder):
     def default(self, o):
         if isinstance(o, types.MappingProxyType):
-            return dict(o)
+            d = dict(o)
+            # Always remove SafeLoader left-over
+            d.pop('_source_context', None)
+            d.pop('_start_mark', None)
+            return d
         return json.JSONEncoder.default(self, o)
 
 
diff --git a/zuul/rpclistener.py b/zuul/rpclistener.py
index c97a9cf9d4..5c0294771e 100644
--- a/zuul/rpclistener.py
+++ b/zuul/rpclistener.py
@@ -17,7 +17,6 @@ import json
 import logging
 import threading
 import traceback
-import types
 
 import gear
 
@@ -25,13 +24,7 @@ from zuul import model
 from zuul.connection import BaseConnection
 from zuul.lib import encryption
 from zuul.lib.config import get_default
-
-
-class MappingProxyEncoder(json.JSONEncoder):
-    def default(self, obj):
-        if isinstance(obj, types.MappingProxyType):
-            return dict(obj)
-        return json.JSONEncoder.default(self, obj)
+from zuul.lib.jsonutil import ZuulJSONEncoder
 
 
 class RPCListener(object):
@@ -361,7 +354,7 @@ class RPCListener(object):
         output = []
         for job in jobs:
             output.append(job.toDict(tenant))
-        gear_job.sendWorkComplete(json.dumps(output, cls=MappingProxyEncoder))
+        gear_job.sendWorkComplete(json.dumps(output, cls=ZuulJSONEncoder))
 
     def handle_job_list(self, job):
         args = json.loads(job.arguments)
@@ -431,7 +424,7 @@ class RPCListener(object):
                 config['pipelines'].append(pipeline)
             result['configs'].append(config)
 
-        gear_job.sendWorkComplete(json.dumps(result, cls=MappingProxyEncoder))
+        gear_job.sendWorkComplete(json.dumps(result, cls=ZuulJSONEncoder))
 
     def handle_project_list(self, job):
         args = json.loads(job.arguments)