diff --git a/src/constants.cc b/src/constants.cc
index c32f136..65af2fb 100644
--- a/src/constants.cc
+++ b/src/constants.cc
@@ -18,6 +18,18 @@
 
 namespace art {
 
+std::ostream& operator<<(std::ostream& os, const InstructionSet& rhs) {
+  switch (rhs) {
+    case kNone: os << "none"; break;
+    case kArm: os << "ARM"; break;
+    case kThumb2: os << "Thumb2"; break;
+    case kX86: os << "x86"; break;
+    case kMips: os << "MIPS"; break;
+    default: os << "InstructionSet[" << static_cast<int>(rhs) << "]"; break;
+  }
+  return os;
+}
+
 std::ostream& operator<<(std::ostream& os, const InvokeType& rhs) {
   switch (rhs) {
     case kStatic: os << "static"; break;
diff --git a/src/constants.h b/src/constants.h
index bb17c2d..832d6f7 100644
--- a/src/constants.h
+++ b/src/constants.h
@@ -29,6 +29,8 @@
   kMips
 };
 
+std::ostream& operator<<(std::ostream& os, const InstructionSet& rhs);
+
 enum InvokeType {
   kStatic, kDirect, kVirtual, kSuper, kInterface,
   kMaxInvokeType = kInterface
diff --git a/src/disassembler.cc b/src/disassembler.cc
index ae781dc..5e3f500 100644
--- a/src/disassembler.cc
+++ b/src/disassembler.cc
@@ -27,8 +27,7 @@
   if (instruction_set == kArm || instruction_set == kThumb2) {
     return new arm::DisassemblerArm();
   } else {
-    // TODO: give a better fatal message
-    UNIMPLEMENTED(FATAL);
+    UNIMPLEMENTED(FATAL) << "no disassembler for " << instruction_set;
     return NULL;
   }
 }