Index: lib/MC/MCParser/AsmParser.cpp
===================================================================
--- lib/MC/MCParser/AsmParser.cpp
+++ lib/MC/MCParser/AsmParser.cpp
@@ -1842,7 +1842,7 @@
   // Otherwise, we have a normal instruction or directive.
 
   // Directives start with "."
-  if (IDVal[0] == '.' && IDVal != ".") {
+  if (IDVal.size() && IDVal[0] == '.' && IDVal != ".") {
     // There are several entities interested in parsing directives:
     //
     // 1. The target-specific assembly parser. Some directives are target
Index: test/Assembler/empty-string.s
===================================================================
--- /dev/null
+++ test/Assembler/empty-string.s
@@ -0,0 +1,3 @@
+// REQUIRES: riscv-registered-target
+// RUN: not llvm-mc -triple riscv32-unknown-linux-gnu < %s 2>&1 | FileCheck %s
+"" # CHECK: error: unrecognized instruction mnemonic