diff --git a/gcc/fortran/interface.cc b/gcc/fortran/interface.cc
index 64b90550be2aa655220e3f472549a47345c98a0a..7b86a338bc15f5934eb0310d31cb4e5476c0344a 100644
--- a/gcc/fortran/interface.cc
+++ b/gcc/fortran/interface.cc
@@ -3439,6 +3439,10 @@ gfc_compare_actual_formal (gfc_actual_arglist **ap, gfc_formal_arglist *formal,
       if (f->sym->ts.type == BT_CLASS)
 	goto skip_size_check;
 
+      /* Skip size check for NULL() actual without MOLD argument.  */
+      if (a->expr->expr_type == EXPR_NULL && a->expr->ts.type == BT_UNKNOWN)
+	goto skip_size_check;
+
       actual_size = get_expr_storage_size (a->expr);
       formal_size = get_sym_storage_size (f->sym);
       if (actual_size != 0 && actual_size < formal_size
diff --git a/gcc/testsuite/gfortran.dg/null_actual_5.f90 b/gcc/testsuite/gfortran.dg/null_actual_5.f90
new file mode 100644
index 0000000000000000000000000000000000000000..1198715b7c8d6b85a6475328c978e4509b850737
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/null_actual_5.f90
@@ -0,0 +1,76 @@
+! { dg-do compile }
+! PR fortran/55978
+!
+! Passing of NULL() with and without MOLD as actual argument
+!
+! Testcase derived from pr55978 comment#16
+
+program pr55978_c16
+  implicit none
+
+  integer, pointer       :: p(:)
+  integer, allocatable   :: a(:)
+  character(10), pointer :: c
+  character(10), pointer :: cp(:)
+
+  type t
+    integer, pointer     :: p(:)
+    integer, allocatable :: a(:)
+  end type
+
+  type(t) :: d
+
+  ! (1) pointer
+  p => null()
+  call sub (p)
+
+  ! (2) allocatable
+  call sub (a)
+  call sub (d%a)
+
+  ! (3) pointer component
+  d%p => null ()
+  call sub (d%p)
+
+  ! (4) NULL
+  call sub (null (a))   ! OK
+  call sub (null (p))   ! OK
+  call sub (null (d%a)) ! OK
+  call sub (null (d%p)) ! OK
+  call sub (null ())    ! was erroneously rejected with:
+  ! Actual argument contains too few elements for dummy argument 'x' (1/4)
+
+  call bla (null(c))
+  call bla (null())     ! was erroneously rejected with:
+  ! Actual argument contains too few elements for dummy argument 'x' (1/10)
+
+  call foo (null(cp))
+  call foo (null())
+
+  call bar (null(cp))
+  call bar (null())     ! was erroneously rejected with:
+  ! Actual argument contains too few elements for dummy argument 'x' (1/70)
+
+contains
+
+  subroutine sub(x)
+    integer, intent(in), optional :: x(4)
+    if (present (x)) stop 1
+  end
+
+  subroutine bla(x)
+    character(len=10), intent(in), optional :: x
+    if (present (x)) stop 2
+  end
+
+  subroutine foo(x)
+    character(len=10), intent(in), optional :: x(:)
+    if (present (x)) stop 3
+  end
+
+  subroutine bar(x)
+    character(len=10), intent(in), optional :: x(7)
+    if (present (x)) stop 4
+  end
+
+end