diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 1a3cc6c30a..8fd128ac6f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2018-10-03  Tom Tromey  <tom@tromey.com>
+
+	* expression.h (enum exp_opcode): Use uint8_t as base type.
+	* expprint.c (op_name): Handle invalid opcodes.
+
 2018-10-03  Tom Tromey  <tom@tromey.com>
 
 	* parse.c (prefixify_expression): Add assert.
diff --git a/gdb/expprint.c b/gdb/expprint.c
index d6ed41253e..e87b3b709b 100644
--- a/gdb/expprint.c
+++ b/gdb/expprint.c
@@ -687,6 +687,13 @@ static int dump_subexp_body (struct expression *exp, struct ui_file *, int);
 const char *
 op_name (struct expression *exp, enum exp_opcode opcode)
 {
+  if (opcode >= OP_UNUSED_LAST)
+    {
+      char *cell = get_print_cell ();
+      xsnprintf (cell, PRINT_CELL_SIZE, "unknown opcode: %u",
+		 unsigned (opcode));
+      return cell;
+    }
   return exp->language_defn->la_exp_desc->op_name (opcode);
 }
 
diff --git a/gdb/expression.h b/gdb/expression.h
index bc7625f984..a5cb4c678e 100644
--- a/gdb/expression.h
+++ b/gdb/expression.h
@@ -39,7 +39,7 @@
    and skip that many.  Strings, like numbers, are indicated
    by the preceding opcode.  */
 
-enum exp_opcode
+enum exp_opcode : uint8_t
   {
 #define OP(name) name ,