diff --git a/compiler/ml_code_util.m b/compiler/ml_code_util.m
index 69b5a4e84..360966cb2 100644
--- a/compiler/ml_code_util.m
+++ b/compiler/ml_code_util.m
@@ -423,10 +423,10 @@
 %
 
 :- pred ml_generate_constants_for_arms(list(prog_var)::in, list(hlds_goal)::in,
-    list(list(mlds_rval))::out, ml_gen_info::in, ml_gen_info::out) is det.
+    list(list(mlds_rval))::out, ml_gen_info::in, ml_gen_info::out) is semidet.
 
 :- pred ml_generate_constants_for_arm(list(prog_var)::in, hlds_goal::in,
-    list(mlds_rval)::out, ml_gen_info::in, ml_gen_info::out) is det.
+    list(mlds_rval)::out, ml_gen_info::in, ml_gen_info::out) is semidet.
 
 :- pred ml_generate_field_assign(mlds_lval::in, mlds_type::in,
     mlds_field_id::in, mlds_vector_common::in, mlds_type::in,
@@ -1656,24 +1656,14 @@ ml_generate_constants_for_arm(Vars, Goal, Soln, !Info) :-
     ml_gen_info_get_const_var_map(!.Info, InitConstVarMap),
     ml_gen_goal(model_det, Goal, _LocalVarDefns, _FuncDefns, _Stmts, !Info),
     ml_gen_info_get_const_var_map(!.Info, FinalConstVarMap),
-    list.map(lookup_ground_rval(FinalConstVarMap), Vars, Soln),
+    list.map(search_ground_rval(FinalConstVarMap), Vars, Soln),
     ml_gen_info_set_const_var_map(InitConstVarMap, !Info).
 
-:- pred lookup_ground_rval(ml_ground_term_map::in, prog_var::in,
-    mlds_rval::out) is det.
+:- pred search_ground_rval(ml_ground_term_map::in, prog_var::in,
+    mlds_rval::out) is semidet.
 
-lookup_ground_rval(FinalConstVarMap, Var, Rval) :-
-    % We can do a map.lookup instead of a map.search here because
-    % - we execute this code only if we have already determined that
-    %   goal_is_conj_of_unify succeeds for this arm,
-    % - we don't even start looking for lookup switches unless we know
-    %   that the mark_static_terms pass has been run, and
-    % - for every arm on which goal_is_conj_of_unify succeeds,
-    %   mark_static_terms will mark all the variables to which Var
-    %   may be bound as being constructed statically. (There can be no need
-    %   to construct them dynamically, since all the arm's nonlocals are
-    %   output, which means none of them can be input.)
-    map.lookup(FinalConstVarMap, Var, GroundTerm),
+search_ground_rval(FinalConstVarMap, Var, Rval) :-
+    map.search(FinalConstVarMap, Var, GroundTerm),
     GroundTerm = ml_ground_term(Rval, _, _).
 
 ml_generate_field_assign(OutVarLval, FieldType, FieldId, VectorCommon,
diff --git a/compiler/ml_disj_gen.m b/compiler/ml_disj_gen.m
index 774fb7779..93bb87193 100644
--- a/compiler/ml_disj_gen.m
+++ b/compiler/ml_disj_gen.m
@@ -172,8 +172,7 @@ ml_gen_disj(Disjuncts, GoalInfo, CodeModel, Context, Stmts, !Info) :-
                 StaticGroundCells = yes,
 
                 DisjNonLocals = goal_info_get_nonlocals(GoalInfo),
-                all_disjuncts_are_conj_of_unify(DisjNonLocals, Disjuncts)
-            then
+                all_disjuncts_are_conj_of_unify(DisjNonLocals, Disjuncts),
                 % Since the MLDS backend implements trailing by a
                 % HLDS-to-HLDS transform (which is in add_trail_ops.m),
                 % if we get here, then trailing is not enabled, and we do
@@ -182,7 +181,8 @@ ml_gen_disj(Disjuncts, GoalInfo, CodeModel, Context, Stmts, !Info) :-
                 NonLocals = goal_info_get_nonlocals(GoalInfo),
                 OutVars = set_of_var.to_sorted_list(NonLocals),
                 list.map_foldl(ml_generate_constants_for_arm(OutVars),
-                    Disjuncts, Solns, !Info),
+                    Disjuncts, Solns, !Info)
+            then
                 ml_gen_lookup_disj(OutVars, Solns, Context, Stmts, !Info)
             else
                 ml_gen_ordinary_model_non_disj(FirstDisjunct, LaterDisjuncts,
diff --git a/tests/valid/Mmakefile b/tests/valid/Mmakefile
index 5ee4abccc..ace470327 100644
--- a/tests/valid/Mmakefile
+++ b/tests/valid/Mmakefile
@@ -87,6 +87,7 @@ OTHER_PROGS = \
 	bug457 \
 	bug480 \
 	bug480a \
+	bug481 \
 	bug51 \
 	bug85 \
 	builtin_false \
diff --git a/tests/valid/bug481.m b/tests/valid/bug481.m
new file mode 100644
index 000000000..8f646327d
--- /dev/null
+++ b/tests/valid/bug481.m
@@ -0,0 +1,38 @@
+%---------------------------------------------------------------------------%
+% vim: ft=mercury ts=4 sw=4 et
+%---------------------------------------------------------------------------%
+%
+% Before 2019 aug 19, the compiler aborted when t compiling this in hlc.gc.
+% The problem was a map.lookup that tried to look up the variable String's
+% entry in the constant variable map. Since String is not a constant,
+% the lookup aborted. The MLDS code generator had code to ensure that
+% the switch (or disjunction) arms on which it tried to do this consisted
+% only of conjunctions of unifications which had only its outputs as its
+% nonlocal variables, but this code had a bug, which allowed the attempt
+% to generate a lookup table to proceed if, beside its outputs, an arm
+% also had the switched-on variable as a nonlocal. The second arm of the
+% switch on String below uses String in exactly such a manner, triggering
+% the bug.
+
+:- module bug481.
+
+:- interface.
+
+:- type result
+    --->    none
+    ;       enabled(string).
+
+:- pred parse(string::in, result::out) is semidet.
+
+:- implementation.
+
+parse(String, Res) :-
+    (
+        String = "none",
+        Res = none
+    ;
+        ( String = "enable"
+        ; String = "full"
+        ),
+        Res = enabled(String)
+    ).