diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index 73c0256f2c1f5..6b5caff27e8f0 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -81,6 +81,7 @@ pub struct Symbol(u32);
 
 // The interner in thread-local, so `Symbol` shouldn't move between threads.
 impl !Send for Symbol { }
+impl !Sync for Symbol { }
 
 impl Symbol {
     /// Maps a string to its interned representation.