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());
   }