diff --git a/src/mzscheme/gc2/gc2.h b/src/mzscheme/gc2/gc2.h
index 7d4f1ce111..04a5be7cbd 100644
--- a/src/mzscheme/gc2/gc2.h
+++ b/src/mzscheme/gc2/gc2.h
@@ -376,6 +376,10 @@ GC2_EXTERN void GC_write_barrier(void *p);
    Explicit write barrier to ensure that a write-barrier signal is not
    triggered by a memory write.
 */
+GC2_EXTERN void GC_switch_in_master_gc();
+/*
+   Makes the current thread the master GC thread.
+*/
 GC2_EXTERN void GC_switch_out_master_gc();
 /*
    Makes the current GC the master GC.