diff --git a/TODO b/TODO
index 47e9c11d..215307e1 100644
--- a/TODO
+++ b/TODO
@@ -1,7 +1,5 @@
 Working TODO list:
 
- - Cyc_sum, etc: can we pass the actual arg(s) to the error function?
-
  - make *trace-level* a command-line parameter, and by default do not emit scheme code transformations in comments
  - Reduction in size of generated code
    is there anything we can do?
diff --git a/runtime.h b/runtime.h
index 8a884f67..b4af8a90 100644
--- a/runtime.h
+++ b/runtime.h
@@ -797,7 +797,10 @@ static common_type FUNC_OP(object x, object y) { \
     } else if (tx == double_tag && ty == double_tag) { \
         s.double_t.value = ((double_type *)x)->value OP ((double_type *)y)->value; \
     } else { \
-        Cyc_rt_raise_msg("Bad argument type\n"); \
+        make_string(s, "Bad argument type"); \
+        make_cons(c1, y, nil); \
+        make_cons(c0, &s, &c1); \
+        Cyc_rt_raise(&c0); \
     } \
     return s; \
 } \
@@ -839,7 +842,10 @@ static common_type Cyc_num_op_va_list(int argc, common_type (fn_op(object, objec
     sum.double_t.tag = double_tag;
     sum.double_t.value = ((double_type *)n)->value;
   } else {
-      Cyc_rt_raise_msg("Bad argument type");
+      make_string(s, "Bad argument type");
+      make_cons(c1, n, nil);
+      make_cons(c0, &s, &c1);
+      Cyc_rt_raise(&c0);
   }
 
   for (i = 1; i < argc; i++) {
@@ -851,7 +857,7 @@ static common_type Cyc_num_op_va_list(int argc, common_type (fn_op(object, objec
         sum.double_t.tag = double_tag;
         sum.double_t.value = ((double_type *) &result)->value;
     } else {
-        Cyc_rt_raise_msg("Invalid tag in Cyc_num_op_va_list");
+        Cyc_rt_raise_msg("Internal error, invalid tag in Cyc_num_op_va_list");
     }
   }