diff --git a/py/mpstate.h b/py/mpstate.h
index 18bec09..f1f8fd9 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -29,6 +29,7 @@
 #include <stdint.h>
 
 #include "py/mpconfig.h"
+#include "py/mpthread.h"
 #include "py/misc.h"
 #include "py/nlr.h"
 #include "py/obj.h"
@@ -80,6 +81,11 @@
     #if MICROPY_PY_GC_COLLECT_RETVAL
     size_t gc_collected;
     #endif
+
+    #if MICROPY_PY_THREAD
+    // This is a global mutex used to make the GC thread-safe.
+    mp_thread_mutex_t gc_mutex;
+    #endif
 } mp_state_mem_t;
 
 // This structure hold runtime and VM information.  It includes a section