diff --git a/inline.h b/inline.h
index 80a7b76878..6e3efedee6 100644
--- a/inline.h
+++ b/inline.h
@@ -1040,9 +1040,9 @@ Indicates whether the stacked SV C<sv> (assumed to be not yet popped off
 the stack) is only kept alive due to a single reference from the argument
 stack and/or and the temps stack.
 
-This can used for example to decide whether the copying of return values in rvalue
-context can be skipped, or whether it shouldn't be assigned to in lvalue
-context.
+This can used for example to decide whether the copying of return values
+in rvalue context can be skipped, or whether it shouldn't be assigned to
+in lvalue context.
 
 =cut
 */
@@ -1063,6 +1063,7 @@ Perl_rpp_is_lone(pTHX_ SV *sv)
     return SvREFCNT(sv) <= cBOOL(SvTEMP(sv))
 #ifdef PERL_RC_STACK
                          + 1
+            && !SvIMMORTAL(sv) /* PL_sv_undef etc are never stealable */
 #endif
     ;
 }
diff --git a/sv.h b/sv.h
index 2cd4ea2593..53b310eac3 100644
--- a/sv.h
+++ b/sv.h
@@ -2415,7 +2415,17 @@ that already have a PV buffer allocated, but no SvTHINKFIRST.
 
 #ifdef DEBUGGING
    /* exercise the immortal resurrection code in sv_free2() */
-#  define SvREFCNT_IMMORTAL 1000
+#  ifdef PERL_RC_STACK
+     /* When the stack is ref-counted, the code tends to take a lot of
+      * short cuts with immortals, such as skipping the bump of the ref
+      * count of PL_sv_undef when pushing it on the stack. Exercise that
+      * this doesn't cause problems, especially on code which
+      * special-cases RC==1 etc.
+      */
+#    define SvREFCNT_IMMORTAL 10
+#  else
+#    define SvREFCNT_IMMORTAL 1000
+#  endif
 #else
 #  define SvREFCNT_IMMORTAL ((~(U32)0)/2)
 #endif
diff --git a/t/op/hash.t b/t/op/hash.t
index 0ad4890824..ee4ff903df 100644
--- a/t/op/hash.t
+++ b/t/op/hash.t
@@ -259,6 +259,27 @@ package Magic {
     ::is(join( ':', %inner), "x:y", "magic keys");
 }
 
+# Make sure PL_sv_undef is copied, and not stored directly, when assigning
+# to a hash. This failed on DEBUGGING + PERL_RC_STACK builds because the
+# test for a lone SV assumed that an SV on the stack with a ref count of 1
+# could be used directly rather than copied. However, PL_sv_undef and
+# friends could reach a ref count of 1 but still not be stealable.
+#
+# A DEBUGGING build sets the initial ref count of the immortals to 1000
+# rather than I32_MAX, making such problems easier to reproduce.
+
+{
+    my $bad = 0;
+    for (1..1001) {
+        # Each iteration may leave the RC of PL_sv_undef one lower.
+        my %h  = ('a', undef);
+        # this could fail with
+        # "Modification of non-creatable hash value attempted ..."
+        eval { $h{a} = 1; };
+        $bad = 1 if $@ ne "";
+    }
+    ok(!$bad, "PL_sv_undef RC 1");
+}
 
 
 done_testing();