diff --git a/util/mutex.h b/util/mutex.h
index 3bb0602..66b11dd 100644
--- a/util/mutex.h
+++ b/util/mutex.h
@@ -15,8 +15,6 @@
 #include <unistd.h>  // For POSIX options
 #endif
 
-namespace re2 {
-
 #if !defined(_WIN32)
   // Possible values of POSIX options:
   //   -1 means not supported,
@@ -71,6 +69,8 @@
 # error Need to implement mutex.h for your architecture, or #define NO_THREADS
 #endif
 
+namespace re2 {
+
 class Mutex {
  public:
   // Create a Mutex that is not held by anybody.