Index: lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp
===================================================================
--- lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp
+++ lib/Target/RISCV/InstPrinter/RISCVInstPrinter.cpp
@@ -101,6 +101,8 @@
     O << 'r';
   if ((FenceArg & RISCVFenceField::W) != 0)
     O << 'w';
+  if (FenceArg == 0)
+    O << "unknown";
 }
 
 void RISCVInstPrinter::printFRMArg(const MCInst *MI, unsigned OpNo,
Index: test/MC/Disassembler/RISCV/unknown-fence-field.txt
===================================================================
--- /dev/null
+++ test/MC/Disassembler/RISCV/unknown-fence-field.txt
@@ -0,0 +1,9 @@
+# RUN: llvm-mc -disassemble -triple=riscv32 -mattr=+f,+d < %s 2>&1 | FileCheck %s
+# RUN: llvm-mc -disassemble -triple=riscv64 -mattr=+f,+d < %s 2>&1 | FileCheck %s
+#
+# Test generated by a LLVM MC Disassembler Protocol Buffer Fuzzer
+# for the RISC-V assembly language.
+
+# This decodes as fence , iorw with invalid fence field as 0.
+[0x0f 0x00 0xf0 0x00]
+# CHECK: fence unknown, iorw