Index: lib/Sema/SemaInit.cpp
===================================================================
--- lib/Sema/SemaInit.cpp
+++ lib/Sema/SemaInit.cpp
@@ -2269,15 +2269,17 @@
           assert(StructuredList->getNumInits() == 1
                  && "A union should never have more than one initializer!");
 
-          // We're about to throw away an initializer, emit warning.
-          SemaRef.Diag(D->getFieldLoc(),
-                       diag::warn_initializer_overrides)
-            << D->getSourceRange();
           Expr *ExistingInit = StructuredList->getInit(0);
-          SemaRef.Diag(ExistingInit->getLocStart(),
-                       diag::note_previous_initializer)
-            << /*FIXME:has side effects=*/0
-            << ExistingInit->getSourceRange();
+          if (ExistingInit) {
+            // We're about to throw away an initializer, emit warning.
+            SemaRef.Diag(D->getFieldLoc(),
+                         diag::warn_initializer_overrides)
+              << D->getSourceRange();
+            SemaRef.Diag(ExistingInit->getLocStart(),
+                         diag::note_previous_initializer)
+              << /*FIXME:has side effects=*/0
+              << ExistingInit->getSourceRange();
+          }
 
           // remove existing initializer
           StructuredList->resizeInits(SemaRef.Context, 0);
Index: test/Sema/designated-initializers.c
===================================================================
--- test/Sema/designated-initializers.c
+++ test/Sema/designated-initializers.c
@@ -351,3 +351,20 @@
   { { 'f', 'o', 'o' }, 1 },
   [0].L[4] = 'x' // no-warning
 };
+
+struct {
+  struct { } s1;
+  union {
+    int a;
+    int b;
+  } u1;
+} s = {
+  .s1 = {
+    .x = 0, // expected-error{{field designator}}
+  },
+
+  .u1 = {
+    .a = 0,
+    .b = 0,
+  },
+};