diff --git a/flang/lib/Lower/IO.cpp b/flang/lib/Lower/IO.cpp
--- a/flang/lib/Lower/IO.cpp
+++ b/flang/lib/Lower/IO.cpp
@@ -599,9 +599,10 @@
                                               const fir::ExtendedValue &item) {
   mlir::Type argType = inputFunc.getFunctionType().getInput(1);
   llvm::SmallVector<mlir::Value> inputFuncArgs = {cookie};
-  if (argType.isa<fir::BoxType>()) {
+  if (argType.isa<fir::BaseBoxType>()) {
     mlir::Value box = fir::getBase(item);
-    assert(box.getType().isa<fir::BoxType>() && "must be previously emboxed");
+    assert(box.getType().isa<fir::BaseBoxType>() &&
+           "must be previously emboxed");
     inputFuncArgs.push_back(builder.createConvert(loc, argType, box));
   } else {
     mlir::Value itemAddr = fir::getBase(item);
diff --git a/flang/test/Lower/polymorphic.f90 b/flang/test/Lower/polymorphic.f90
--- a/flang/test/Lower/polymorphic.f90
+++ b/flang/test/Lower/polymorphic.f90
@@ -10,6 +10,10 @@
     procedure :: print
     procedure :: assign_p1_int
     procedure :: elemental_fct
+    procedure :: read_p1
+    procedure :: write_p1
+    generic :: read(formatted) => read_p1
+    generic :: write(formatted) => write_p1
     generic :: assignment(=) => assign_p1_int
     procedure :: host_assoc
   end type
@@ -543,4 +547,41 @@
 ! CHECK: %{{.*}} = fir.call @_FortranAioOutputDescriptor(%{{.*}}, %[[BOX_NONE]]) fastmath<contract> : (!fir.ref<i8>, !fir.box<none>) -> i1
 ! CHECK: fir.freemem %[[TMP]] : !fir.heap<!fir.array<5x5xi32>>
 
+  subroutine write_p1(dtv, unit, iotype, v_list, iostat, iomsg)
+    class(p1), intent(in) :: dtv
+    integer, intent(in) :: unit
+    character(*), intent(in) :: iotype
+    integer, intent(in) :: v_list(:)
+    integer, intent(out) :: iostat
+    character(*), intent(inout) :: iomsg
+    ! dummy subroutine for testing purpose
+  end subroutine
+
+  subroutine read_p1(dtv, unit, iotype, v_list, iostat, iomsg)
+    class(p1), intent(inout) :: dtv
+    integer, intent(in) :: unit
+    character(*), intent(in) :: iotype
+    integer, intent(in) :: v_list(:)
+    integer, intent(out) :: iostat
+    character(*), intent(inout) :: iomsg
+    ! dummy subroutine for testing purpose
+  end subroutine
+
+  subroutine test_polymorphic_io()
+    type(p1), target :: t
+    class(p1), pointer :: p
+    open(17, form='formatted', access='stream')
+    write(17, 1) t
+    1 Format(1X,I10)
+    p => t
+    rewind(17)
+    read(17, 1) p
+  end subroutine
+
+! CHECK-LABEL: func.func @_QMpolymorphic_testPtest_polymorphic_io() {
+! CHECK: %[[P:.*]] = fir.alloca !fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>> {bindc_name = "p", uniq_name = "_QMpolymorphic_testFtest_polymorphic_ioEp"}
+! CHECK: %[[LOAD_P:.*]] = fir.load %[[P]] : !fir.ref<!fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>>>
+! CHECK: %[[BOX_NONE:.*]] = fir.convert %[[LOAD_P]] : (!fir.class<!fir.ptr<!fir.type<_QMpolymorphic_testTp1{a:i32,b:i32}>>>) -> !fir.box<none>
+! CHECK: %{{.*}} = fir.call @_FortranAioInputDescriptor(%{{.*}}, %[[BOX_NONE]]) {{.*}} : (!fir.ref<i8>, !fir.box<none>) -> i1
+
 end module