Index: lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
===================================================================
--- lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -2477,14 +2477,9 @@
     return MatchOperand_ParseFail;
   }
 
-  auto DB = AArch64DB::lookupDBByName(Tok.getString());
-  if (!DB) {
-    TokError("invalid barrier option name");
-    return MatchOperand_ParseFail;
-  }
-
   // The only valid named option for ISB is 'sy'
-  if (Mnemonic == "isb" && DB->Encoding != AArch64DB::sy) {
+  auto DB = AArch64DB::lookupDBByName(Tok.getString());
+  if (Mnemonic == "isb" && (!DB || DB->Encoding != AArch64DB::sy)) {
     TokError("'sy' or #imm operand expected");
     return MatchOperand_ParseFail;
   }
Index: test/MC/AArch64/basic-a64-diagnostics.s
===================================================================
--- test/MC/AArch64/basic-a64-diagnostics.s
+++ test/MC/AArch64/basic-a64-diagnostics.s
@@ -3276,12 +3276,16 @@
 
         isb #-1
         isb #16
+        isb sys
 // CHECK-ERROR-NEXT: error: {{Invalid immediate for instruction|barrier operand out of range}}
 // CHECK-ERROR-NEXT:         isb #-1
 // CHECK-ERROR-NEXT:             ^
 // CHECK-ERROR-NEXT: error: {{Invalid immediate for instruction|barrier operand out of range}}
 // CHECK-ERROR-NEXT:         isb #16
 // CHECK-ERROR-NEXT:             ^
+// CHECK-ERROR-NEXT: error: 'sy' or #imm operand expected
+// CHECK-ERROR-NEXT:        isb sys
+// CHECK-ERROR-NEXT:            ^
 
         msr daifset, x4
         msr spsel, #-1