diff --git a/lib/Target/X86/X86InstrInfo.td b/lib/Target/X86/X86InstrInfo.td
index 2fea6e17738..5e03a59adc1 100644
--- a/lib/Target/X86/X86InstrInfo.td
+++ b/lib/Target/X86/X86InstrInfo.td
@@ -1599,7 +1599,8 @@ def CMPXCHG16B : RI<0xC7, MRM1m, (outs), (ins i128mem:$dst),
 def LOCK_PREFIX : I<0xF0, RawFrm, (outs),  (ins), "lock", []>;
 
 // Rex64 instruction prefix
-def REX64_PREFIX : I<0x48, RawFrm, (outs),  (ins), "rex64", []>;
+def REX64_PREFIX : I<0x48, RawFrm, (outs),  (ins), "rex64", []>,
+                     Requires<[In64BitMode]>;
 
 // Data16 instruction prefix
 def DATA16_PREFIX : I<0x66, RawFrm, (outs),  (ins), "data16", []>;
diff --git a/utils/TableGen/X86RecognizableInstr.cpp b/utils/TableGen/X86RecognizableInstr.cpp
index 3deca23c6bc..1198e5a9779 100644
--- a/utils/TableGen/X86RecognizableInstr.cpp
+++ b/utils/TableGen/X86RecognizableInstr.cpp
@@ -276,7 +276,6 @@ RecognizableInstr::RecognizableInstr(DisassemblerTables &tables,
   }
   // FIXME: These instructions aren't marked as 64-bit in any way
   Is64Bit |= Rec->getName() == "JMP64pcrel32" ||
-             Rec->getName() == "REX64_PREFIX" ||
              Rec->getName().find("MOV64") != Name.npos ||
              Rec->getName().find("PUSH64") != Name.npos ||
              Rec->getName().find("POP64") != Name.npos;