diff --git a/pp.c b/pp.c
index 77dddcb8b3..c7fa23189d 100644
--- a/pp.c
+++ b/pp.c
@@ -4300,7 +4300,8 @@ PP(pp_uc)
 		    SvCUR_set(dest, len);
 		    len = sv_utf8_upgrade_flags_grow(dest,
 						SV_GMAGIC|SV_FORCE_UTF8_UPGRADE,
-						extra);
+                                                extra
+                                              + 1 /* trailing NUL */ );
 		    d = (U8*)SvPVX(dest) + len;
 
                     /* Now process the remainder of the source, simultaneously
@@ -4434,7 +4435,8 @@ PP(pp_lc)
              * when converted to UTF-8 */
             sv_utf8_upgrade_flags_grow(dest, 0, len
                                               + I_count
-                                              + variant_under_utf8_count(s, send));
+                                              + variant_under_utf8_count(s, send)
+                                              + 1 /* Trailing NUL */ );
             d = (U8*)SvPVX(dest);
             has_turkic_I = TRUE;
         }
@@ -4803,7 +4805,8 @@ PP(pp_fc)
                     SvCUR_set(dest, len);
                     len = sv_utf8_upgrade_flags_grow(dest,
                                                 SV_GMAGIC|SV_FORCE_UTF8_UPGRADE,
-                                                extra);
+                                                extra
+                                              + 1 /* Trailing NUL */ );
                     d = (U8*)SvPVX(dest) + len;
 
                     *d++ = UTF8_TWO_BYTE_HI(GREEK_SMALL_LETTER_MU);
diff --git a/t/uni/upper.t b/t/uni/upper.t
index 252b51ce39..3c8d8c2be7 100644
--- a/t/uni/upper.t
+++ b/t/uni/upper.t
@@ -11,8 +11,9 @@ use feature 'unicode_strings';
 
 is(uc("\x{3B1}\x{345}\x{301}"), "\x{391}\x{301}\x{399}",
                                                    'Verify moves YPOGEGRAMMENI');
+fresh_perl_is('use 5.026;m.\U00ÿÿ0000.', "", {}, "[perl #133876]  This caused valgrind and asan errors");
 
-casetest( 1,	# extra tests already run
+casetest( 2,	# extra tests already run
 	"Uppercase_Mapping",
 	 uc                        => sub { uc $_[0] },
 	 uc_with_appended_null_arg => sub { my $a = ""; uc ($_[0] . $a) }