Index: lld/trunk/ELF/Driver.cpp
===================================================================
--- lld/trunk/ELF/Driver.cpp
+++ lld/trunk/ELF/Driver.cpp
@@ -1029,7 +1029,10 @@
       StringRef From;
       StringRef To;
       std::tie(From, To) = StringRef(Arg->getValue()).split('=');
-      readDefsym(From, MemoryBufferRef(To, "-defsym"));
+      if (From.empty() || To.empty())
+        error("-defsym: syntax error: " + StringRef(Arg->getValue()));
+      else
+        readDefsym(From, MemoryBufferRef(To, "-defsym"));
       break;
     }
     case OPT_script:
Index: lld/trunk/test/ELF/defsym.s
===================================================================
--- lld/trunk/test/ELF/defsym.s
+++ lld/trunk/test/ELF/defsym.s
@@ -74,6 +74,12 @@
 # RUN: not ld.lld -o %t %t.o --defsym=xxx=yyy,zzz 2>&1 | FileCheck %s -check-prefix=ERR2
 # ERR2: -defsym:1: EOF expected, but got ,
 
+# RUN: not ld.lld -o %t %t.o --defsym=foo 2>&1 | FileCheck %s -check-prefix=ERR3
+# ERR3: error: -defsym: syntax error: foo
+
+# RUN: not ld.lld -o %t %t.o --defsym= 2>&1 | FileCheck %s -check-prefix=ERR4
+# ERR4: error: -defsym: syntax error:
+
 .globl foo1
  foo1 = 0x123