diff --git a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
--- a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
@@ -4002,6 +4002,19 @@
   EXPECT_EQ(Second.activeParameter, 1);
 }
 
+TEST(CompletionTest, NoCrashTests) {
+  llvm::StringLiteral Cases[] = {
+      R"cpp(
+    template <typename = int> struct Foo {};
+    auto a = [x(3)](Foo<^>){};
+    )cpp",
+  };
+  for (auto Case : Cases) {
+    SCOPED_TRACE(Case);
+    auto Completions = completions(Case);
+  }
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang
diff --git a/clang/lib/Index/USRGeneration.cpp b/clang/lib/Index/USRGeneration.cpp
--- a/clang/lib/Index/USRGeneration.cpp
+++ b/clang/lib/Index/USRGeneration.cpp
@@ -262,6 +262,12 @@
 
   // Mangle in type information for the arguments.
   for (auto *PD : D->parameters()) {
+    // FIXME: Make sure we don't have nullptrs in parameter lists of function
+    // decls.
+    if (!PD) {
+      IgnoreResults = true;
+      return;
+    }
     Out << '#';
     VisitType(PD->getType());
   }