Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -1026,7 +1026,10 @@
       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");
+      else
+        readDefsym(From, MemoryBufferRef(To, "-defsym"));
       break;
     }
     case OPT_script:
Index: test/ELF/defsym.s
===================================================================
--- test/ELF/defsym.s
+++ test/ELF/defsym.s
@@ -68,8 +68,12 @@
 # EXPR-NEXT:   Section: Absolute
 # EXPR-NEXT: }
 
-# RUN: not ld.lld -o %t %t.o --defsym=foo2=und 2>&1 | FileCheck %s -check-prefix=ERR
-# ERR: error: -defsym:1: symbol not found: und
+# RUN: not ld.lld -o %t %t.o --defsym=foo2=und 2>&1 | FileCheck %s -check-prefix=ERR1
+# ERR1: error: -defsym:1: symbol not found: und
+
+# RUN: not ld.lld -o %t %t.o --defsym= 2>&1 | FileCheck %s -check-prefix=ERR2
+# RUN: not ld.lld -o %t %t.o --defsym=foo 2>&1 | FileCheck %s -check-prefix=ERR2
+# ERR2: error: -defsym: syntax error
 
 .globl foo1
  foo1 = 0x123