Index: flang/lib/Evaluate/check-expression.cpp
===================================================================
--- flang/lib/Evaluate/check-expression.cpp
+++ flang/lib/Evaluate/check-expression.cpp
@@ -175,6 +175,12 @@
   template <typename T> bool operator()(const Expr<T> &x) {
     return std::visit([=](const auto &y) { return (*this)(y); }, x.u);
   }
+  bool operator()(const Expr<SomeType> &x) {
+    if (IsNullPointer(x)) {
+      return true;
+    }
+    return std::visit([this](const auto &y) { return (*this)(y); }, x.u);
+  }
   template <typename A> bool operator()(const A *x) { return x && (*this)(*x); }
   template <typename A> bool operator()(const std::optional<A> &x) {
     return x && (*this)(*x);
Index: flang/lib/Semantics/expression.cpp
===================================================================
--- flang/lib/Semantics/expression.cpp
+++ flang/lib/Semantics/expression.cpp
@@ -1766,7 +1766,8 @@
                 *symbol);
           }
         } else if (IsAllocatable(*symbol) && IsBareNullPointer(&*value)) {
-          // NULL() with no arguments allowed by 7.5.10 para 6 for ALLOCATABLE
+          // NULL() with no arguments allowed by 7.5.10 para 6 for ALLOCATABLE.
+          result.Add(*symbol, Expr<SomeType>{NullPointer{}});
         } else if (auto symType{DynamicType::From(symbol)}) {
           if (IsAllocatable(*symbol) && symType->IsUnlimitedPolymorphic() &&
               valueType) {
@@ -1795,10 +1796,12 @@
   // Ensure that unmentioned component objects have default initializers.
   for (const Symbol &symbol : components) {
     if (!symbol.test(Symbol::Flag::ParentComp) &&
-        unavailable.find(symbol.name()) == unavailable.cend() &&
-        !IsAllocatable(symbol)) {
-      if (const auto *details{
-              symbol.detailsIf<semantics::ObjectEntityDetails>()}) {
+        unavailable.find(symbol.name()) == unavailable.cend()) {
+      if (IsAllocatable(symbol)) {
+        // Set all remaining allocatables to explicit NULL()
+        result.Add(symbol, Expr<SomeType>{NullPointer{}});
+      } else if (const auto *details{
+                     symbol.detailsIf<semantics::ObjectEntityDetails>()}) {
         if (details->init()) {
           result.Add(symbol, common::Clone(*details->init()));
         } else { // C799
Index: flang/test/Semantics/modfile45.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/modfile45.f90
@@ -0,0 +1,24 @@
+! RUN: %python %S/test_modfile.py %s %flang_fc1
+! Ensures that uninitialized allocatable components in a structure constructor
+! appear with explicit NULL() in the expression representation.
+module m
+  type t
+    real, allocatable :: x1, x2, x3
+  end type
+  type t2
+    type(t) :: a = t(NULL(),x2=NULL())
+  end type
+end module
+
+!Expect: m.mod
+!module m
+!type::t
+!real(4),allocatable::x1
+!real(4),allocatable::x2
+!real(4),allocatable::x3
+!end type
+!type::t2
+!type(t)::a=t(x1=NULL(),x2=NULL(),x3=NULL())
+!end type
+!intrinsic::null
+!end
Index: flang/test/Semantics/oldparam02.f90
===================================================================
--- flang/test/Semantics/oldparam02.f90
+++ flang/test/Semantics/oldparam02.f90
@@ -12,6 +12,7 @@
   !CHECK: error: Must be a constant value
   parameter p2 = x2
   !CHECK: error: Whole assumed-size array 'x3' may not appear here without subscripts
+  !CHECK: error: Must be a constant value
   parameter p3 = x3
   !CHECK: error: Must be a constant value
   parameter p4 = x4