diff --git a/unittests/DebugInfo/PDB/MappedBlockStreamTest.cpp b/unittests/DebugInfo/PDB/MappedBlockStreamTest.cpp
index 0ed74a0f6a6..2d7814e55b6 100644
--- a/unittests/DebugInfo/PDB/MappedBlockStreamTest.cpp
+++ b/unittests/DebugInfo/PDB/MappedBlockStreamTest.cpp
@@ -63,7 +63,7 @@ public:
   Error commit() const override { return Error::success(); }
 
   MsfStreamLayout layout() const {
-    return MsfStreamLayout{Data.size(), Blocks};
+    return MsfStreamLayout{static_cast<uint32_t>(Data.size()), Blocks};
   }
 
 private: