diff --git a/embed.fnc b/embed.fnc
index 551d46880562..648545ac1691 100644
--- a/embed.fnc
+++ b/embed.fnc
@@ -3095,7 +3095,7 @@ S	|int	|sv_2iuv_non_preserve	|NN SV *const sv
 SR	|STRLEN	|expect_number	|NN const char **const pattern
 ST	|STRLEN	|sv_pos_u2b_forwards|NN const U8 *const start \
 		|NN const U8 *const send|NN STRLEN *const uoffset \
-		|NN bool *const at_end
+		|NN bool *const at_end|NN bool *canonical_position
 ST	|STRLEN	|sv_pos_u2b_midway|NN const U8 *const start \
 		|NN const U8 *send|STRLEN uoffset|const STRLEN uend
 S	|STRLEN	|sv_pos_u2b_cached|NN SV *const sv|NN MAGIC **const mgp \
diff --git a/proto.h b/proto.h
index faca6d1366e1..56f7d42e7e0c 100644
--- a/proto.h
+++ b/proto.h
@@ -6382,9 +6382,9 @@ STATIC STRLEN	S_sv_pos_b2u_midway(pTHX_ const U8 *const s, const U8 *const targe
 STATIC STRLEN	S_sv_pos_u2b_cached(pTHX_ SV *const sv, MAGIC **const mgp, const U8 *const start, const U8 *const send, STRLEN uoffset, STRLEN uoffset0, STRLEN boffset0);
 #define PERL_ARGS_ASSERT_SV_POS_U2B_CACHED	\
 	assert(sv); assert(mgp); assert(start); assert(send)
-STATIC STRLEN	S_sv_pos_u2b_forwards(const U8 *const start, const U8 *const send, STRLEN *const uoffset, bool *const at_end);
+STATIC STRLEN	S_sv_pos_u2b_forwards(const U8 *const start, const U8 *const send, STRLEN *const uoffset, bool *const at_end, bool *canonical_position);
 #define PERL_ARGS_ASSERT_SV_POS_U2B_FORWARDS	\
-	assert(start); assert(send); assert(uoffset); assert(at_end)
+	assert(start); assert(send); assert(uoffset); assert(at_end); assert(canonical_position)
 STATIC STRLEN	S_sv_pos_u2b_midway(const U8 *const start, const U8 *send, STRLEN uoffset, const STRLEN uend);
 #define PERL_ARGS_ASSERT_SV_POS_U2B_MIDWAY	\
 	assert(start); assert(send)
diff --git a/sv.c b/sv.c
index ceef9966da34..eec33eb5cab1 100644
--- a/sv.c
+++ b/sv.c
@@ -7225,7 +7225,8 @@ Perl_sv_len_utf8_nomg(pTHX_ SV * const sv)
    offset.  */
 static STRLEN
 S_sv_pos_u2b_forwards(const U8 *const start, const U8 *const send,
-		      STRLEN *const uoffset_p, bool *const at_end)
+		      STRLEN *const uoffset_p, bool *const at_end,
+		      bool* canonical_position)
 {
     const U8 *s = start;
     STRLEN uoffset = *uoffset_p;
@@ -7245,6 +7246,9 @@ S_sv_pos_u2b_forwards(const U8 *const start, const U8 *const send,
 	   it's actually a bounds error  */
 	s = send;
     }
+    /* If the unicode position is beyond the end, we return the end but
+       shouldn't cache that position */
+    *canonical_position = (uoffset == 0);
     *uoffset_p -= uoffset;
     return s - start;
 }
@@ -7298,6 +7302,7 @@ S_sv_pos_u2b_cached(pTHX_ SV *const sv, MAGIC **const mgp, const U8 *const start
     STRLEN boffset = 0; /* Actually always set, but let's keep gcc happy.  */
     bool found = FALSE;
     bool at_end = FALSE;
+    bool canonical_position = FALSE;
 
     PERL_ARGS_ASSERT_SV_POS_U2B_CACHED;
 
@@ -7338,7 +7343,8 @@ S_sv_pos_u2b_cached(pTHX_ SV *const sv, MAGIC **const mgp, const U8 *const start
 		    uoffset -= uoffset0;
 		    boffset = boffset0
 			+ sv_pos_u2b_forwards(start + boffset0,
-					      send, &uoffset, &at_end);
+					      send, &uoffset, &at_end,
+					      &canonical_position);
 		    uoffset += uoffset0;
 		}
 	    }
@@ -7380,7 +7386,8 @@ S_sv_pos_u2b_cached(pTHX_ SV *const sv, MAGIC **const mgp, const U8 *const start
 	STRLEN real_boffset;
 	uoffset -= uoffset0;
 	real_boffset = boffset0 + sv_pos_u2b_forwards(start + boffset0,
-						      send, &uoffset, &at_end);
+						      send, &uoffset, &at_end,
+						      &canonical_position);
 	uoffset += uoffset0;
 
 	if (found && PL_utf8cache < 0)
@@ -7389,7 +7396,7 @@ S_sv_pos_u2b_cached(pTHX_ SV *const sv, MAGIC **const mgp, const U8 *const start
 	boffset = real_boffset;
     }
 
-    if (PL_utf8cache && !SvGMAGICAL(sv) && SvPOK(sv)) {
+    if (PL_utf8cache && canonical_position && !SvGMAGICAL(sv) && SvPOK(sv)) {
 	if (at_end)
 	    utf8_mg_len_cache_update(sv, mgp, uoffset);
 	else
diff --git a/t/op/index.t b/t/op/index.t
index 2f0834e8d1ec..a218848851e9 100644
--- a/t/op/index.t
+++ b/t/op/index.t
@@ -8,7 +8,7 @@ BEGIN {
 }
 
 use strict;
-plan( tests => 414 );
+plan( tests => 415 );
 
 run_tests() unless caller;
 
@@ -358,4 +358,12 @@ $x;
 EOS
     }
 
+    {
+        my $s = "abc";
+        my $len = length($s);
+        utf8::upgrade($s);
+        length($s);
+        is(index($s, "", $len+1), 3, 'Overlong index doesn\'t confuse utf8 cache');
+    }
+
 } # end of sub run_tests