diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
@@ -644,6 +644,10 @@
 
       auto format = TARGET_OS_OSX ? "10.%d" : "%d.0";
       len = internal_snprintf(vers, len, format, os_major);
+    } else if (internal_strlen(vers) < 3) {
+      // Fallback for cases when sanitizer initialization runs before sysctl is
+      // setup.  Assume we run on a "very new" version of the OS.
+      len = internal_snprintf(vers, sizeof(VersStr), "99.0");
     }
   }
   CHECK_LT(len, sizeof(VersStr));