diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 58776e8705b6..e8764a0edf57 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -5651,6 +5651,9 @@ static void checkDLLAttributeRedeclaration(Sema &S, NamedDecl *OldDecl,
                                            NamedDecl *NewDecl,
                                            bool IsSpecialization,
                                            bool IsDefinition) {
+  if(OldDecl->isInvalidDecl())
+    return;
+
   if (TemplateDecl *OldTD = dyn_cast<TemplateDecl>(OldDecl)) {
     OldDecl = OldTD->getTemplatedDecl();
     if (!IsSpecialization)
diff --git a/clang/test/Sema/dllimport.c b/clang/test/Sema/dllimport.c
index 0728cf14a8e3..a7fb00e3f773 100644
--- a/clang/test/Sema/dllimport.c
+++ b/clang/test/Sema/dllimport.c
@@ -210,6 +210,10 @@ __declspec(dllimport) void redecl6();
                       void redecl7();
 __declspec(dllimport) inline void redecl7() {}
 
+// PR31069: Don't crash trying to merge attributes for redeclaration of invalid decl.
+void __declspec(dllimport) redecl8(unknowntype X); // expected-error{{unknown type name 'unknowntype'}}
+void redecl8(unknowntype X) { } // expected-error{{unknown type name 'unknowntype'}}
+
 // External linkage is required.
 __declspec(dllimport) static int staticFunc(); // expected-error{{'staticFunc' must have external linkage when declared 'dllimport'}}