diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index eb629dc44c156..3744746f1531a 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -1233,7 +1233,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "prints the llvm optimization passes being run"),
     ast_json: bool = (false, parse_bool, [UNTRACKED],
         "print the AST as JSON and halt"),
-    threads: Option<usize> = (None, parse_opt_uint, [UNTRACKED],
+    threads: Option<usize> = (Some(1), parse_opt_uint, [UNTRACKED],
         "use a thread pool with N threads"),
     ast_json_noexpand: bool = (false, parse_bool, [UNTRACKED],
         "print the pre-expansion AST as JSON and halt"),