diff --git a/lib/CodeGen/RegAllocFast.cpp b/lib/CodeGen/RegAllocFast.cpp
index 6a5282cbbbf..17d9492d942 100644
--- a/lib/CodeGen/RegAllocFast.cpp
+++ b/lib/CodeGen/RegAllocFast.cpp
@@ -193,9 +193,10 @@ namespace {
     void spillVirtReg(MachineBasicBlock::iterator MI, unsigned VirtReg);
 
     void usePhysReg(MachineOperand &MO);
-    void definePhysReg(MachineInstr &MI, MCPhysReg PhysReg, RegState NewState);
+    void definePhysReg(MachineBasicBlock::iterator MI, MCPhysReg PhysReg,
+                       RegState NewState);
     unsigned calcSpillCost(MCPhysReg PhysReg) const;
-    void assignVirtToPhysReg(LiveReg&, MCPhysReg PhysReg);
+    void assignVirtToPhysReg(LiveReg &, MCPhysReg PhysReg);
 
     LiveRegMap::iterator findLiveVirtReg(unsigned VirtReg) {
       return LiveVirtRegs.find(TargetRegisterInfo::virtReg2Index(VirtReg));
@@ -434,8 +435,8 @@ void RegAllocFast::usePhysReg(MachineOperand &MO) {
 /// Mark PhysReg as reserved or free after spilling any virtregs. This is very
 /// similar to defineVirtReg except the physreg is reserved instead of
 /// allocated.
-void RegAllocFast::definePhysReg(MachineInstr &MI, MCPhysReg PhysReg,
-                                 RegState NewState) {
+void RegAllocFast::definePhysReg(MachineBasicBlock::iterator MI,
+                                 MCPhysReg PhysReg, RegState NewState) {
   markRegUsedInInstr(PhysReg);
   switch (unsigned VirtReg = PhysRegState[PhysReg]) {
   case regDisabled:
@@ -857,7 +858,7 @@ void RegAllocFast::allocateBasicBlock(MachineBasicBlock &MBB) {
   // Add live-in registers as live.
   for (const MachineBasicBlock::RegisterMaskPair LI : MBB.liveins())
     if (MRI->isAllocatable(LI.PhysReg))
-      definePhysReg(*MII, LI.PhysReg, regReserved);
+      definePhysReg(MII, LI.PhysReg, regReserved);
 
   VirtDead.clear();
   Coalesced.clear();
diff --git a/test/CodeGen/AArch64/fast-regalloc-empty-bb-with-liveins.mir b/test/CodeGen/AArch64/fast-regalloc-empty-bb-with-liveins.mir
new file mode 100644
index 00000000000..fc19173a176
--- /dev/null
+++ b/test/CodeGen/AArch64/fast-regalloc-empty-bb-with-liveins.mir
@@ -0,0 +1,26 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64-apple-ios -run-pass regallocfast -o - %s | FileCheck %s
+# This test used to crash the fast register alloc.
+# Basically, when a basic block has liveins, the fast regalloc
+# was deferencing the begin iterator of this block. However,
+# when this block is empty and it will just crashed!
+---
+name:            crashing
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: crashing
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   liveins: %x0, %x1
+  ; CHECK: bb.1:
+  ; CHECK:   renamable %w0 = MOVi32imm -1
+  ; CHECK:   RET_ReallyLR implicit killed %w0
+  bb.1:
+    liveins: %x0, %x1
+
+  bb.2:
+    %0:gpr32 = MOVi32imm -1
+    %w0 = COPY %0
+    RET_ReallyLR implicit %w0
+
+...