diff --git a/compiler-rt/lib/dfsan/dfsan_custom.cpp b/compiler-rt/lib/dfsan/dfsan_custom.cpp
--- a/compiler-rt/lib/dfsan/dfsan_custom.cpp
+++ b/compiler-rt/lib/dfsan/dfsan_custom.cpp
@@ -1573,6 +1573,7 @@
                        dfsan_label *ret_label) {
   int ret = sigemptyset(set);
   dfsan_set_label(0, set, sizeof(sigset_t));
+  *ret_label = 0;
   return ret;
 }