diff --git a/src/mudlib/embedded_if_session.py b/src/mudlib/embedded_if_session.py
index 490fcd7..139c947 100644
--- a/src/mudlib/embedded_if_session.py
+++ b/src/mudlib/embedded_if_session.py
@@ -165,7 +165,8 @@ class EmbeddedIFSession:
         except Exception as e:
             tb = traceback.format_exc()
             logger.error(f"Interpreter crashed:\n{tb}")
-            self._error = f"interpreter error: {e}"
+            msg = str(e) or type(e).__name__
+            self._error = f"interpreter error: {msg}"
         finally:
             self._done = True
             self._keyboard._waiting.set()
diff --git a/src/mudlib/zmachine/zcpu.py b/src/mudlib/zmachine/zcpu.py
index 0053577..4778bb4 100644
--- a/src/mudlib/zmachine/zcpu.py
+++ b/src/mudlib/zmachine/zcpu.py
@@ -226,17 +226,27 @@ class ZCpu:
         return table
 
     def step_fast(self):
-        """Execute a single instruction without tracing.
+        """Execute a single instruction with lightweight tracing.
 
         Returns True if execution should continue.
         """
+        current_pc = self._opdecoder.program_counter
         (opcode_class, opcode_number, operands) = self._opdecoder.get_next_instruction()
         entry = self._dispatch[opcode_class][opcode_number]
         if entry is None:
-            raise ZCpuIllegalInstruction
+            self._trace.append(f"  {current_pc:06x}  ILLEGAL")
+            self._dump_trace()
+            raise ZCpuIllegalInstruction(
+                f"illegal opcode class={opcode_class} num={opcode_number}"
+                f" at PC={current_pc:#x}"
+            )
         implemented, func = entry
         if not implemented:
             return False
+        self._trace.append(
+            f"  {current_pc:06x}  {func.__name__}"
+            f"({', '.join(str(x) for x in operands)})"
+        )
         try:
             func(self, *operands)
         except (ZCpuQuit, ZCpuRestart):