diff --git a/flang/include/flang/Optimizer/Dialect/FIRType.h b/flang/include/flang/Optimizer/Dialect/FIRType.h
--- a/flang/include/flang/Optimizer/Dialect/FIRType.h
+++ b/flang/include/flang/Optimizer/Dialect/FIRType.h
@@ -318,6 +318,9 @@
 /// value.
 bool isUnlimitedPolymorphicType(mlir::Type ty);
 
+/// Return true iff `ty` is the type of an assumed type.
+bool isAssumedType(mlir::Type ty);
+
 /// Return true iff `boxTy` wraps a record type or an unlimited polymorphic
 /// entity. Polymorphic entities with intrinsic type spec do not have addendum
 inline bool boxHasAddendum(fir::BaseBoxType boxTy) {
diff --git a/flang/lib/Lower/ConvertExpr.cpp b/flang/lib/Lower/ConvertExpr.cpp
--- a/flang/lib/Lower/ConvertExpr.cpp
+++ b/flang/lib/Lower/ConvertExpr.cpp
@@ -2715,12 +2715,12 @@
             box = Fortran::evaluate::IsVariable(*expr)
                       ? builder.createBox(loc, genBoxArg(*expr),
                                           fir::isPolymorphicType(argTy),
-                                          fir::isBoxNone(argTy))
+                                          fir::isAssumedType(argTy))
                       : builder.createBox(getLoc(), genTempExtAddr(*expr),
                                           fir::isPolymorphicType(argTy),
-                                          fir::isBoxNone(argTy));
+                                          fir::isAssumedType(argTy));
             if (box.getType().isa<fir::BoxType>() &&
-                fir::isPolymorphicType(argTy) && !fir::isBoxNone(argTy)) {
+                fir::isPolymorphicType(argTy) && !fir::isAssumedType(argTy)) {
               mlir::Type actualTy = argTy;
               if (Fortran::lower::isParentComponent(*expr))
                 actualTy = fir::BoxType::get(converter.genType(*expr));
diff --git a/flang/lib/Optimizer/Builder/FIRBuilder.cpp b/flang/lib/Optimizer/Builder/FIRBuilder.cpp
--- a/flang/lib/Optimizer/Builder/FIRBuilder.cpp
+++ b/flang/lib/Optimizer/Builder/FIRBuilder.cpp
@@ -360,10 +360,11 @@
     return create<fir::EmboxProcOp>(loc, toTy, proc);
   }
 
-  if ((fir::isPolymorphicType(fromTy) &&
+ if (((fir::isPolymorphicType(fromTy) &&
        (fir::isAllocatableType(fromTy) || fir::isPointerType(fromTy)) &&
        fir::isPolymorphicType(toTy)) ||
-      (fir::isPolymorphicType(fromTy) && toTy.isa<fir::BoxType>()))
+      (fir::isPolymorphicType(fromTy) && toTy.isa<fir::BoxType>())) &&
+      !(fir::isUnlimitedPolymorphicType(fromTy) && fir::isAssumedType(toTy)))
     return create<fir::ReboxOp>(loc, toTy, val, mlir::Value{},
                                 /*slice=*/mlir::Value{});
 
diff --git a/flang/lib/Optimizer/Dialect/FIRType.cpp b/flang/lib/Optimizer/Dialect/FIRType.cpp
--- a/flang/lib/Optimizer/Dialect/FIRType.cpp
+++ b/flang/lib/Optimizer/Dialect/FIRType.cpp
@@ -300,7 +300,7 @@
   return false;
 }
 
-static bool isAssumedType(mlir::Type ty) {
+bool isAssumedType(mlir::Type ty) {
   if (auto boxTy = ty.dyn_cast<fir::BoxType>()) {
     if (boxTy.getEleTy().isa<mlir::NoneType>())
       return true;