Index: SemaInit.cpp
===================================================================
--- SemaInit.cpp
+++ SemaInit.cpp
@@ -2260,15 +2260,17 @@
           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: sema-segvcheck.c
===================================================================
--- sema-segvcheck.c
+++ sema-segvcheck.c
@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only %s; test $? -eq 1
+
+typedef struct {
+  union {
+    unsigned long long house;
+    struct {
+      unsigned cat1;
+      unsigned cat2;
+    };
+  };
+} struct_0;
+
+
+typedef struct {
+  union {
+    struct {
+      union {
+        unsigned cows;
+        struct {
+          unsigned char c:1;
+        };
+      };
+    };
+  };
+
+  union {
+    struct {
+      unsigned bird0;
+      unsigned bird1;
+    };
+  };
+} struct_1;
+
+
+typedef struct {
+  struct_0 s0;
+  struct_1 s1[1];
+} struct_2;
+
+struct_2 s = {
+  .s0 = {
+    .dog = 0x00000000, // expected-error{{field designator}}
+  },
+
+  .s1[0] = {
+    .cows = 0x00005050,
+    .c = 1,
+  },
+};