diff --git a/src/script_opt/ZAM/Expr.cc b/src/script_opt/ZAM/Expr.cc
index 881fd35441..27fdae8ff3 100644
--- a/src/script_opt/ZAM/Expr.cc
+++ b/src/script_opt/ZAM/Expr.cc
@@ -1100,16 +1100,34 @@ const ZAMStmt ZAMCompiler::ConstructSet(const NameExpr* n, const Expr* e) {
 
 const ZAMStmt ZAMCompiler::ConstructRecord(const NameExpr* n, const Expr* e) {
     auto rc = e->AsRecordConstructorExpr();
+    auto rt = e->GetType()->AsRecordType();
+    auto map = rc->Map();
 
     ZInstI z;
 
-    if ( rc->Map() ) {
-        z = GenInst(OP_CONSTRUCT_KNOWN_RECORD_V, n);
+    if ( map ) {
+        // Compute whether we need at least one of the default field
+        // initializations. We don't need so if every one of the fields
+        // with such an initializer is already covered in the map (meaning
+        // it has an explicit initialization value).
+        auto& ci = rt->CreationInits();
+        zeek_uint_t common = 0;
+        for ( auto& c : ci )
+            for ( auto r : *map )
+                if ( c.first == r )
+                    ++common;
+
+        auto need_init = common != ci.size();
+
+        z = GenInst(OP_CONSTRUCT_KNOWN_RECORD_Vi, n, need_init);
         z.aux = InternalBuildVals(rc->Op().get());
-        z.aux->map = *rc->Map();
+        z.aux->map = *map;
     }
     else {
-        z = GenInst(OP_CONSTRUCT_RECORD_V, n);
+        // Constructors that don't need maps are explicitly initializing
+        // every field, so they don't need default initializations, hence
+        // "false" in the following.
+        z = GenInst(OP_CONSTRUCT_DIRECT_RECORD_Vi, n, false);
         z.aux = InternalBuildVals(rc->Op().get());
     }
 
@@ -1118,6 +1136,33 @@ const ZAMStmt ZAMCompiler::ConstructRecord(const NameExpr* n, const Expr* e) {
     if ( pfs->HasSideEffects(SideEffectsOp::CONSTRUCTION, z.t) )
         z.aux->can_change_non_locals = true;
 
+    // If one of the initialization values is an unspecified vector (which
+    // in general we can't know until run-time) then we'll need to
+    // "concretize" it. We first see whether this is a possibility, since
+    // it usually isn't, by counting up how many of the record fields are
+    // vectors.
+    std::vector<int> vector_fields; // holds indices of the vector fields
+    for ( int i = 0; i < z.aux->n; ++i ) {
+        auto field_ind = map ? (*map)[i] : i;
+        if ( rt->GetFieldType(field_ind)->Tag() == TYPE_VECTOR )
+            vector_fields.push_back(field_ind);
+    }
+
+    auto inst = AddInst(z);
+
+    if ( vector_fields.empty() )
+        // Common case of no vector fields, we're done.
+        return inst;
+
+    // Need to add a separate instruction for concretizing the fields.
+    z = GenInst(OP_CONCRETIZE_VECTOR_FIELDS_V, n);
+    z.t = e->GetType();
+    int nf = static_cast<int>(vector_fields.size());
+    z.aux = new ZInstAux(nf);
+    z.aux->elems_has_slots = false; // we're storing field offsets, not slots
+    for ( int i = 0; i < nf; ++i )
+        z.aux->Add(i, vector_fields[i]);
+
     return AddInst(z);
 }
 
diff --git a/src/script_opt/ZAM/Ops.in b/src/script_opt/ZAM/Ops.in
index 00dbbaebdb..699007f771 100644
--- a/src/script_opt/ZAM/Ops.in
+++ b/src/script_opt/ZAM/Ops.in
@@ -1199,35 +1199,49 @@ eval	ConstructTableOrSetPre()
 
 direct-unary-op Record-Constructor ConstructRecord
 
-internal-op Construct-Record
-type V
+# v2 is boolean of whether to initialize fields. For this instruction,
+# it's always false.
+op Construct-Direct-Record
+type Vi
 eval	EvalConstructRecord(, i)
 
+# v2 is boolean of whether to initialize fields.
+op Construct-Known-Record
+type Vi
+eval	EvalConstructRecord(auto& map = aux->map;, map[i])
+
 macro EvalConstructRecord(map_init, map_accessor)
 	auto rt = cast_intrusive<RecordType>(z.t);
-	auto new_r = new RecordVal(rt);
+	auto new_r = new RecordVal(rt, z.v2 ? RecordVal::RV_FULL_INIT : RecordVal::RV_SLOTS_INIT);
 	auto aux = z.aux;
 	auto n = aux->n;
 	map_init
 	for ( auto i = 0; i < n; ++i )
-		{
-		auto v_i = aux->ToVal(frame, i);
-		auto ind = map_accessor;
-		if ( v_i && v_i->GetType()->Tag() == TYPE_VECTOR &&
-		     v_i->GetType<VectorType>()->IsUnspecifiedVector() )
-			{
-			const auto& t_ind = rt->GetFieldType(ind);
-			v_i->AsVectorVal()->Concretize(t_ind->Yield());
-			}
-		new_r->Assign(ind, v_i);
-		}
+		new_r->InitField(map_accessor, aux->ToZVal(frame, i));
 	auto& r = frame[z.v1].record_val;
 	Unref(r);
 	r = new_r;
 
-internal-op Construct-Known-Record
+# Special instruction for concretizing vectors that are fields in a
+# newly-constructed record. "aux" holds which fields in the record to
+# inspect.
+op Concretize-Vector-Fields
+op1-read
 type V
-eval	EvalConstructRecord(auto& map = aux->map;, map[i])
+eval	auto rt = cast_intrusive<RecordType>(z.t);
+	auto r = frame[z.v1].record_val;
+	auto aux = z.aux;
+	auto n = aux->n;
+	for ( auto i = 0; i < n; ++i )
+		{
+		auto v_i = r->GetField(aux->elems[i].IntVal());
+		ASSERT(v_i);
+		if ( v_i->GetType<VectorType>()->IsUnspecifiedVector() )
+			{
+			const auto& t_i = rt->GetFieldType(i);
+			v_i->AsVectorVal()->Concretize(t_i->Yield());
+			}
+		}
 
 direct-unary-op Vector-Constructor ConstructVector
 
diff --git a/src/script_opt/ZAM/ZInst.h b/src/script_opt/ZAM/ZInst.h
index 9e1c840d7c..41854fe2c6 100644
--- a/src/script_opt/ZAM/ZInst.h
+++ b/src/script_opt/ZAM/ZInst.h
@@ -418,6 +418,9 @@ public:
     // and type.
     void Add(int i, int slot, TypePtr t) { elems[i].SetInt(slot, t); }
 
+    // Same, but for non-slot integers.
+    void Add(int i, int v_i) { elems[i].SetInt(v_i); }
+
     // Same but for constants.
     void Add(int i, ValPtr c) { elems[i].SetConstant(c); }