diff --git a/migen/fhdl/tools.py b/migen/fhdl/tools.py
index 9a6c3a668..685f4df37 100644
--- a/migen/fhdl/tools.py
+++ b/migen/fhdl/tools.py
@@ -256,12 +256,13 @@ def rename_clock_domain_expr(f, old, new):
 
 def rename_clock_domain(f, old, new):
     rename_clock_domain_expr(f, old, new)
-    if old in f.sync:
-        if new in f.sync:
-            f.sync[new].extend(f.sync[old])
-        else:
-            f.sync[new] = f.sync[old]
-        del f.sync[old]
+    if new != old:
+        if old in f.sync:
+            if new in f.sync:
+                f.sync[new].extend(f.sync[old])
+            else:
+                f.sync[new] = f.sync[old]
+            del f.sync[old]
     for special in f.specials:
         special.rename_clock_domain(old, new)
     try: