diff --git a/lib/Sema/SemaDeclAttr.cpp b/lib/Sema/SemaDeclAttr.cpp
index 8e2bac6ed0f69b6844bdddbe62c0ce382f7ed90e..5f6f81de148f4c1f8efdaef7b4a473969a7c3077 100644
--- a/lib/Sema/SemaDeclAttr.cpp
+++ b/lib/Sema/SemaDeclAttr.cpp
@@ -3441,7 +3441,7 @@ void Sema::CheckAlignasUnderalignment(Decl *D) {
     Ty = VD->getType();
   else
     Ty = Context.getTagDeclType(cast<TagDecl>(D));
-  if (Ty->isDependentType())
+  if (Ty->isDependentType() || Ty->isIncompleteType())
     return;
 
   // C++11 [dcl.align]p5, C11 6.7.5/4:
diff --git a/test/CXX/dcl.dcl/dcl.attr/dcl.align/p6.cpp b/test/CXX/dcl.dcl/dcl.attr/dcl.align/p6.cpp
index 8cf5b266bb25973b164b46404b65d50c99364569..e7885774807708a84cbf90da8f76e514dab6ae10 100644
--- a/test/CXX/dcl.dcl/dcl.attr/dcl.align/p6.cpp
+++ b/test/CXX/dcl.dcl/dcl.attr/dcl.align/p6.cpp
@@ -81,3 +81,6 @@ enum alignas(O) alignas(P) Y<M,N,O,P>::E : char { e };
 int y1848 = Y<1,8,4,8>::e;
 // FIXME: We should reject this.
 int y1248 = Y<1,2,4,8>::e;
+
+// Don't crash here.
+alignas(4) struct Incomplete incomplete; // expected-error {{incomplete type}} expected-note {{forward declaration}}