diff --git a/gcc/fortran/interface.c b/gcc/fortran/interface.c
index 320eb01..71721dd 100644
--- a/gcc/fortran/interface.c
+++ b/gcc/fortran/interface.c
@@ -484,13 +484,24 @@ gfc_compare_types (gfc_typespec *ts1, gfc_typespec *ts2)
   if (ts1->type == BT_VOID || ts2->type == BT_VOID)
     return 1;
 
-  if (ts1->type == BT_CLASS
-      && ts1->u.derived->components->ts.u.derived->attr.unlimited_polymorphic)
+  /* The _data component is not always present, therefore check for its
+     presence before assuming, that its derived->attr is available.
+     When the _data component is not present, then nevertheless the
+     unlimited_polymorphic flag may be set in the derived type's attr.  */
+  if (ts1->type == BT_CLASS && ts1->u.derived->components
+      && ((strcmp (ts1->u.derived->components->name, "_data") == 0
+	   && ts1->u.derived->components->ts.u.derived->attr
+						  .unlimited_polymorphic)
+	  || ts1->u.derived->attr.unlimited_polymorphic))
     return 1;
 
   /* F2003: C717  */
   if (ts2->type == BT_CLASS && ts1->type == BT_DERIVED
-      && ts2->u.derived->components->ts.u.derived->attr.unlimited_polymorphic
+      && ts2->u.derived->components
+      && ((strcmp (ts2->u.derived->components->name, "_data") == 0
+	   && ts2->u.derived->components->ts.u.derived->attr
+						  .unlimited_polymorphic)
+	  || ts2->u.derived->attr.unlimited_polymorphic)
       && (ts1->u.derived->attr.sequence || ts1->u.derived->attr.is_bind_c))
     return 1;
 
diff --git a/gcc/fortran/symbol.c b/gcc/fortran/symbol.c
index 44392e8..ac1a0b1 100644
--- a/gcc/fortran/symbol.c
+++ b/gcc/fortran/symbol.c
@@ -4567,7 +4567,10 @@ gfc_type_compatible (gfc_typespec *ts1, gfc_typespec *ts2)
 
   if (is_class1
       && ts1->u.derived->components
-      && ts1->u.derived->components->ts.u.derived->attr.unlimited_polymorphic)
+      && ((strcmp (ts1->u.derived->components->name, "_data") == 0
+	   && ts1->u.derived->components->ts.u.derived->attr
+							.unlimited_polymorphic)
+	  || ts1->u.derived->attr.unlimited_polymorphic))
     return 1;
 
   if (!is_derived1 && !is_derived2 && !is_class1 && !is_class2)
@@ -4576,13 +4579,14 @@ gfc_type_compatible (gfc_typespec *ts1, gfc_typespec *ts2)
   if (is_derived1 && is_derived2)
     return gfc_compare_derived_types (ts1->u.derived, ts2->u.derived);
 
-  if (is_derived1 && is_class2)
+  if (is_derived1 && is_class2 && ts2->u.derived->components)
     return gfc_compare_derived_types (ts1->u.derived,
 				      ts2->u.derived->components->ts.u.derived);
-  if (is_class1 && is_derived2)
+  if (is_class1 && is_derived2 && ts1->u.derived->components)
     return gfc_type_is_extension_of (ts1->u.derived->components->ts.u.derived,
 				     ts2->u.derived);
-  else if (is_class1 && is_class2)
+  else if (is_class1 && is_class2 && ts1->u.derived->components
+	   && ts2->u.derived->components)
     return gfc_type_is_extension_of (ts1->u.derived->components->ts.u.derived,
 				     ts2->u.derived->components->ts.u.derived);
   else
diff --git a/gcc/testsuite/gfortran.dg/implicit_class_1.f90 b/gcc/testsuite/gfortran.dg/implicit_class_1.f90
index 329f57a..fff1f2b 100644
--- a/gcc/testsuite/gfortran.dg/implicit_class_1.f90
+++ b/gcc/testsuite/gfortran.dg/implicit_class_1.f90
@@ -4,6 +4,10 @@
 !
 ! Contributed by Reinhold Bader <Reinhold.Bader@lrz.de>
 
+! Add dump-tree-original to check, if the patch preventing a gfortran
+! segfault is working correctly.
+! { dg-options "-fdump-tree-original" }
+
 program upimp
   implicit class(foo) (a-b)
   implicit class(*) (c)
@@ -33,3 +37,5 @@ contains
     implicit class(*) (a-z)
   end
 end program
+
+! { dg-final { cleanup-tree-dump "original" } }