diff --git a/clang-tools-extra/clangd/index/FileIndex.cpp b/clang-tools-extra/clangd/index/FileIndex.cpp
--- a/clang-tools-extra/clangd/index/FileIndex.cpp
+++ b/clang-tools-extra/clangd/index/FileIndex.cpp
@@ -174,9 +174,9 @@
   // not have been indexed, see SymbolCollector::processRelations for details.
   if (Index.Relations) {
     for (const auto &R : *Index.Relations) {
-      auto *File = SymbolIDToFile.lookup(R.Subject);
-      assert(File && "unknown subject in relation");
-      File->Relations.insert(&R);
+      // FIXME: RelationSlab shouldn't contain dangling relations.
+      if (auto *File = SymbolIDToFile.lookup(R.Subject))
+        File->Relations.insert(&R);
     }
   }
   // Store only the direct includes of a file in a shard.
diff --git a/clang-tools-extra/clangd/unittests/FileIndexTests.cpp b/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
--- a/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
@@ -540,6 +540,8 @@
     B.insert(Relation{Sym1.ID, RelationKind::BaseOf, Sym2.ID});
     // Should be stored in b.h
     B.insert(Relation{Sym2.ID, RelationKind::BaseOf, Sym1.ID});
+    // Dangling relation should be dropped.
+    B.insert(Relation{symbol("3").ID, RelationKind::BaseOf, Sym1.ID});
     IF.Relations = std::move(B).build();
   }