diff --git a/lib/tsan/rtl/tsan_rtl.cpp b/lib/tsan/rtl/tsan_rtl.cpp
index ff3bb33..db3d945 100644
--- a/lib/tsan/rtl/tsan_rtl.cpp
+++ b/lib/tsan/rtl/tsan_rtl.cpp
@@ -603,8 +603,8 @@
     // Second and subsequent calls map heap.
     if (shadow_end <= ctx->mapped_shadow_end)
       return;
-    if (ctx->mapped_shadow_begin < shadow_begin)
-      ctx->mapped_shadow_begin = shadow_begin;
+    if (!ctx->mapped_shadow_begin || ctx->mapped_shadow_begin > shadow_begin)
+       ctx->mapped_shadow_begin = shadow_begin;
     if (shadow_begin < ctx->mapped_shadow_end)
       shadow_begin = ctx->mapped_shadow_end;
     VPrintf(2, "MapShadow begin/end = (0x%zx-0x%zx)\n",