diff --git a/CHANGELOG.md b/CHANGELOG.md
index 933883a..1e449e2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   resolved to nothing: only the forward forms counted.  A negative count
   now walks the same instruction statements backwards, labels excluded,
   byte-identical with the toolchain.
+- **The arm64 move-wide family reads its immediate as an unsigned
+  pattern.**  `MOVK $(40000<<48)` folds to a negative int64 and was
+  rejected; the toolchain picks the 16-bit lane from the 64-bit bit
+  pattern, so the encoder now does the same, and a zero immediate is
+  rejected where the toolchain rejects it.
 
 ## [0.35.0] - 2026-09-22
 
diff --git a/asm/arm64_assemble.go b/asm/arm64_assemble.go
index a5e57ff..9d53d8a 100644
--- a/asm/arm64_assemble.go
+++ b/asm/arm64_assemble.go
@@ -3069,29 +3069,29 @@ func encodeARM64MoveWide(mnem string, baseOp uint32, ops []*ast.Operand) ([]byte
 	// base, so MOVZ and MOVN come along for free.
 	opc := baseOp >> 29 & 3
 	sf := baseOp >> 31 & 1
-	v := arm64Imm64(ops[0])
-	if v < 0 {
-		return nil, fmt.Errorf("%s: negative immediate %d", mnem, v)
+	// The toolchain's optab case 33, shared by the whole family in both
+	// widths: the immediate is one unsigned 64-bit pattern (a high-lane
+	// constant such as $(40000<<48) arrives negative through int64
+	// folding), it must occupy exactly one 16-bit lane, zero is rejected,
+	// and the W forms cannot reach the top half.
+	u := uint64(arm64Imm64(ops[0]))
+	if u == 0 {
+		return nil, fmt.Errorf("%s: zero immediate cannot be handled", mnem)
 	}
 	hw := -1
-	for i := range 4 {
-		if v>>(uint(i)*16)&0xFFFF != 0 {
-			hw = i
+	for lane := range 4 {
+		if u&^(uint64(0xFFFF)<<(lane*16)) == 0 {
+			hw = lane
 			break
 		}
 	}
 	if hw < 0 {
-		hw = 0 // zero: every chunk is zero, hw = 0 carries it
-	}
-	for i := hw + 1; i < 4; i++ {
-		if v>>(uint(i)*16)&0xFFFF != 0 {
-			return nil, fmt.Errorf("%s: immediate %d does not fit one 16-bit chunk", mnem, v)
-		}
+		return nil, fmt.Errorf("%s: immediate %#x does not fit one 16-bit chunk", mnem, u)
 	}
 	if sf == 0 && hw > 1 {
-		return nil, fmt.Errorf("%s: immediate %d out of range for the 32-bit form", mnem, v)
+		return nil, fmt.Errorf("%s: immediate %#x out of range for the 32-bit form", mnem, u)
 	}
-	return a64wordLE(a64MoveWide(sf, opc, uint32(hw), uint32(v>>uint(hw*16)&0xFFFF), uint32(rd))), nil
+	return a64wordLE(a64MoveWide(sf, opc, uint32(hw), uint32(u>>uint(hw*16)&0xFFFF), uint32(rd))), nil
 }
 
 // ---- Bitfield/EXTR encoding ----
diff --git a/asm/arm64_encode_test.go b/asm/arm64_encode_test.go
index 772eb8c..7bdafc7 100644
--- a/asm/arm64_encode_test.go
+++ b/asm/arm64_encode_test.go
@@ -1051,6 +1051,41 @@ func TestArm64MOVK(t *testing.T) {
 	}
 }
 
+// TestArm64MOVKHighLane pins the shifted high-lane immediate the arm64 test
+// kernels write: $(40000<<48) folds to a negative int64, and the toolchain
+// reads the value as an unsigned 64-bit pattern when it picks the lane.
+func TestArm64MOVKHighLane(t *testing.T) {
+	got := arm64Words(t, "\tMOVK $(40000<<48), R0\n\tMOVK $0x9c40000000000000, R1\n")
+	want := []uint32{
+		0xf2f38800, // MOVK $(40000<<48), R0 (go tool asm: f2f38800)
+		0xf2f38801, // MOVK hw=3
+		0xd65f03c0,
+	}
+	if len(got) != len(want) {
+		t.Fatalf("word count = %d, want %d", len(got), len(want))
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Errorf("word %d = %08x, want %08x", i, got[i], want[i])
+		}
+	}
+}
+
+// TestArm64MoveWideZeroImmediate pins the toolchain's rejection of a zero
+// immediate in the move-wide family (optab case 33: "zero shifts cannot be
+// handled"): every lane is zero, so no hw field can carry it.
+func TestArm64MoveWideZeroImmediate(t *testing.T) {
+	for _, mnem := range []string{"MOVK", "MOVZ", "MOVN"} {
+		f, errs := parser.Parse("test_arm64.s", "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n\t"+mnem+" $0, R0\n\tRET\n")
+		if len(errs) > 0 {
+			t.Fatalf("%s: parse: %v", mnem, errs)
+		}
+		if _, err := AssembleFileARM64(f); err == nil {
+			t.Errorf("%s $0: expected error, got nil", mnem)
+		}
+	}
+}
+
 // TestArm64LoadImm64 tests 64-bit immediate loading.
 func TestArm64LoadImm64(t *testing.T) {
 	src := `#include "textflag.h"