Index: llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -2088,8 +2088,11 @@
           // Parse an op-mask register mark ({%k<NUM>}), which is now to be
           // expected
           unsigned RegNo;
-          if (!ParseRegister(RegNo, StartLoc, StartLoc) &&
+          SMLoc RegLoc;
+          if (!ParseRegister(RegNo, RegLoc, StartLoc) &&
               X86MCRegisterClasses[X86::VK1RegClassID].contains(RegNo)) {
+            if (RegNo == X86::K0)
+              return Error(RegLoc, "Register k0 can't be used as write mask");
             if (!getLexer().is(AsmToken::RCurly))
               return Error(getLexer().getLoc(), "Expected } at this point");
             Operands.push_back(X86Operand::CreateToken("{", StartLoc));
Index: llvm/trunk/test/MC/X86/avx512-err.s
===================================================================
--- llvm/trunk/test/MC/X86/avx512-err.s
+++ llvm/trunk/test/MC/X86/avx512-err.s
@@ -1,7 +1,6 @@
-// RUN: not llvm-mc -triple x86_64-unknown-unknown -mcpu=knl -mattr=+avx512dq -mattr=+avx512f --show-encoding %s 2> %t.err
-// RUN: FileCheck --check-prefix=ERR < %t.err %s
+// RUN: not llvm-mc %s -triple x86_64-unknown-unknown -mcpu=knl -mattr=+avx512dq -mattr=+avx512f --show-encoding -o /dev/null 2>&1 | FileCheck --check-prefix=ERR %s
 
-// ERR: invalid operand for instruction
+// ERR: Register k0 can't be used as write mask
 vpcmpd $1, %zmm24, %zmm7, %k5{%k0}
 
 // ERR: Expected a {z} mark at this point
Index: llvm/trunk/test/MC/X86/intel-syntax-avx512-error.s
===================================================================
--- llvm/trunk/test/MC/X86/intel-syntax-avx512-error.s
+++ llvm/trunk/test/MC/X86/intel-syntax-avx512-error.s
@@ -7,3 +7,6 @@
 // CHECK: error: Expected a {z} mark at this point
   vfmsub213ps zmm8{rn-sae}, zmm8, zmm8
 // CHECK: error: Expected an op-mask register at this point
+  vpcmpltd k5{k0}, zmm7, zmm24
+// CHECK: error: Register k0 can't be used as write mask
+