Index: gcc/fortran/resolve.c
===================================================================
--- gcc/fortran/resolve.c	(revision 275715)
+++ gcc/fortran/resolve.c	(working copy)
@@ -7487,7 +7487,7 @@ conformable_arrays (gfc_expr *e1, gfc_expr *e2)
   for (tail = e2->ref; tail && tail->next; tail = tail->next);
 
   /* First compare rank.  */
-  if ((tail && e1->rank != tail->u.ar.as->rank)
+  if ((tail && (!tail->u.ar.as || e1->rank != tail->u.ar.as->rank))
       || (!tail && e1->rank != e2->rank))
     {
       gfc_error ("Source-expr at %L must be scalar or have the "
Index: gcc/testsuite/gfortran.dg/pr91727.f90
===================================================================
--- gcc/testsuite/gfortran.dg/pr91727.f90	(nonexistent)
+++ gcc/testsuite/gfortran.dg/pr91727.f90	(working copy)
@@ -0,0 +1,9 @@
+! { dg-do compile }
+! Code contributed by Gerhard Steinmetz.
+program p
+   type t
+      class(*), allocatable :: a
+   end type
+   type(t) :: x
+   allocate (x%a, source=[1]) ! { dg-error "have the same rank as" }
+end