diff --git a/llvm/lib/CodeGen/StackProtector.cpp b/llvm/lib/CodeGen/StackProtector.cpp
--- a/llvm/lib/CodeGen/StackProtector.cpp
+++ b/llvm/lib/CodeGen/StackProtector.cpp
@@ -170,7 +170,8 @@
     // If this instruction accesses memory make sure it doesn't access beyond
     // the bounds of the allocated object.
     Optional<MemoryLocation> MemLoc = MemoryLocation::getOrNone(I);
-    if (MemLoc.hasValue() && MemLoc->Size.getValue() > AllocSize)
+    if (MemLoc.hasValue() && MemLoc->Size.hasValue() &&
+        MemLoc->Size.getValue() > AllocSize)
       return true;
     switch (I->getOpcode()) {
     case Instruction::Store:
diff --git a/llvm/test/CodeGen/X86/stack-guard-memloc-vararg.ll b/llvm/test/CodeGen/X86/stack-guard-memloc-vararg.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/stack-guard-memloc-vararg.ll
@@ -0,0 +1,17 @@
+; RUN: llc -mtriple=x86_64 -O0 < %s | FileCheck %s
+
+; Check that we don't crash on this input.
+; CHECK-LABEL: @foo
+; CHECK: __stack_chk_guard
+; CHECK: retq
+define hidden void @foo(i8** %ptr) #0 {
+entry:
+  %args.addr = alloca i8*, align 8
+  %0 = va_arg i8** %args.addr, i8*
+  store i8* %0, i8** %ptr
+  ret void
+}
+
+attributes #0 = { sspstrong }
+attributes #1 = { optsize }
+