diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -48,6 +48,10 @@
     return Cached->second;
   }
 
+  // If this method is called from an unreachable block, return LoE.
+  if (!MSSA->DT->isReachableFromEntry(BB))
+    return MSSA->getLiveOnEntryDef();
+
   if (BasicBlock *Pred = BB->getSinglePredecessor()) {
     // Single predecessor case, just recurse, we can only have one definition.
     MemoryAccess *Result = getPreviousDefFromEnd(Pred, CachedPreviousDef);
diff --git a/llvm/test/Analysis/MemorySSA/pr43426.ll b/llvm/test/Analysis/MemorySSA/pr43426.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Analysis/MemorySSA/pr43426.ll
@@ -0,0 +1,40 @@
+; RUN: opt -licm -enable-mssa-loop-dependency -S %s | FileCheck %s
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @d()
+define dso_local void @d() {
+entry:
+  br label %header
+
+header:
+  store i32 1, i32* null, align 4
+  br i1 true, label %cleanup53, label %body
+
+body:
+  br i1 undef, label %cleanup31, label %for.cond11
+
+for.cond11: ; Needs branch as is
+  br i1 undef, label %unreachable, label %latch
+
+cleanup31:
+  br label %unreachable
+
+deadblock:
+  br i1 undef, label %unreachable, label %deadblock
+
+cleanup53:
+  %val = load i32, i32* null, align 4
+  %cmpv = icmp eq i32 %val, 0
+  br i1 %cmpv, label %cleanup63, label %latch
+
+latch:
+  br label %header
+
+cleanup63:
+  ret void
+
+unreachable:
+  unreachable
+}
+