diff --git a/base/compiler/ssair/slot2ssa.jl b/base/compiler/ssair/slot2ssa.jl
index b4135fd9dc568..39e092332aae6 100644
--- a/base/compiler/ssair/slot2ssa.jl
+++ b/base/compiler/ssair/slot2ssa.jl
@@ -587,6 +587,7 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, sv::OptimizationState,
     handler_at = compute_trycatch(code, BitSet())
 
     phi_slots = Vector{Int}[Int[] for _ = 1:length(ir.cfg.blocks)]
+    live_slots = Vector{Int}[Int[] for _ = 1:length(ir.cfg.blocks)]
     new_phi_nodes = Vector{NewPhiNode2}[NewPhiNode2[] for _ = 1:length(cfg.blocks)]
     new_phic_nodes = IdDict{Int, Vector{NewPhiCNode2}}()
     for (; leave_block) in catch_entry_blocks
@@ -617,8 +618,10 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, sv::OptimizationState,
             end
             continue
         end
+
         @timeit "liveness" (live = compute_live_ins(cfg, slot))
         for li in live.live_in_bbs
+            push!(live_slots[li], idx)
             cidx = findfirst(x::TryCatchRegion->x.leave_block==li, catch_entry_blocks)
             if cidx !== nothing
                 # The slot is live-in into this block. We need to
@@ -735,7 +738,7 @@ function construct_ssa!(ci::CodeInfo, ir::IRCode, sv::OptimizationState,
         end
         # Record Pi nodes if necessary
         has_pinode = fill(false, length(sv.slottypes))
-        for slot in 1:length(sv.slottypes)
+        for slot in live_slots[item]
             (ival, idef) = incoming_vals[slot]
             (ival === SSAValue(-1)) && continue
             (ival === SSAValue(-2)) && continue
diff --git a/test/compiler/irpasses.jl b/test/compiler/irpasses.jl
index 66d040fe8b893..225839d8e0cf3 100644
--- a/test/compiler/irpasses.jl
+++ b/test/compiler/irpasses.jl
@@ -1519,3 +1519,51 @@ let ir = first(only(Base.code_ircode(f_with_early_try_catch_exit, (); optimize_u
 end
 
 @test isnothing(f_with_early_try_catch_exit())
+
+# Issue #51144 - UndefRefError during compaction
+let m = Meta.@lower 1 + 1
+    @assert Meta.isexpr(m, :thunk)
+    src = m.args[1]::CodeInfo
+    src.code = Any[
+        # block 1  → 2, 3
+        #=  %1: =# Expr(:(=), Core.SlotNumber(4), Core.Argument(2)),
+        #=  %2: =# Expr(:call, :(===), Core.SlotNumber(4), nothing),
+        #=  %3: =# GotoIfNot(Core.SSAValue(1), 5),
+        # block 2
+        #=  %4: =# ReturnNode(nothing),
+        # block 3  → 4, 5
+        #=  %5: =# Expr(:(=), Core.SlotNumber(4), false),
+        #=  %6: =# GotoIfNot(Core.Argument(2), 8),
+        # block 4  → 5
+        #=  %7: =# Expr(:(=), Core.SlotNumber(4), true),
+        # block 5
+        #=  %8: =# ReturnNode(nothing), # Must not insert a π-node here
+    ]
+    nstmts = length(src.code)
+    nslots = 4
+    src.ssavaluetypes = nstmts
+    src.codelocs = fill(Int32(1), nstmts)
+    src.ssaflags = fill(Int32(0), nstmts)
+    src.slotflags = fill(0, nslots)
+    src.slottypes = Any[Any, Union{Bool, Nothing}, Bool, Union{Bool, Nothing}]
+    ir = Core.Compiler.inflate_ir(src)
+
+    mi = ccall(:jl_new_method_instance_uninit, Ref{Core.MethodInstance}, ());
+    mi.specTypes = Tuple{}
+    mi.def = Module()
+
+    # Simulate the important results from inference
+    interp = Core.Compiler.NativeInterpreter()
+    sv = Core.Compiler.OptimizationState(mi, src, interp)
+    slot_id = 4
+    for block_id = 3:5
+        # (_4 !== nothing) conditional narrows the type, triggering PiNodes
+        sv.bb_vartables[block_id][slot_id] = VarState(Bool, #= maybe_undef =# false)
+    end
+
+    ir = Core.Compiler.convert_to_ircode(src, sv)
+    ir = Core.Compiler.slot2reg(ir, src, sv)
+    ir = Core.Compiler.compact!(ir)
+
+    Core.Compiler.verify_ir(ir)
+end