diff --git a/clang-tools-extra/clangd/index/ProjectAware.cpp b/clang-tools-extra/clangd/index/ProjectAware.cpp
--- a/clang-tools-extra/clangd/index/ProjectAware.cpp
+++ b/clang-tools-extra/clangd/index/ProjectAware.cpp
@@ -71,6 +71,8 @@
 };
 
 size_t ProjectAwareIndex::estimateMemoryUsage() const {
+  // Ensure IndexForSpec is up-to-date.
+  getIndex(); // Unused.
   size_t Total = 0;
   std::lock_guard<std::mutex> Lock(Mu);
   for (auto &Entry : IndexForSpec)