diff --git a/ChangeLog b/ChangeLog
index 9ab6986..d676c6b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -55,6 +55,13 @@ patterns.
 10. The error message for an invalid quantifier has been changed from "nothing 
 to repeat" to "quantifier does not follow a repeatable item".
 
+11. If a bad UTF string is compiled with NO_UTF_CHECK, it may succeed, but
+scanning the compiled pattern in subsequent auto-possessification can get out
+of step and lead to an unknown opcode. Previously this could have caused an
+infinite loop. Now it generates an "internal error" error. This is a tidyup,
+not a bug fix; passing bad UTF with NO_UTF_CHECK is documented as having an
+undefined outcome.
+
 
 Version 10.00 05-January-2015
 -----------------------------
diff --git a/src/pcre2_auto_possess.c b/src/pcre2_auto_possess.c
index f531398..0e050e6 100644
--- a/src/pcre2_auto_possess.c
+++ b/src/pcre2_auto_possess.c
@@ -1090,17 +1090,20 @@ but some compilers complain about an unreachable statement. */
 *************************************************/
 
 /* Replaces single character iterations with their possessive alternatives
-if appropriate. This function modifies the compiled opcode!
+if appropriate. This function modifies the compiled opcode! Hitting a 
+non-existant opcode may indicate a bug in PCRE2, but it can also be caused if a 
+bad UTF string was compiled with PCRE2_NO_UTF_CHECK.
 
 Arguments:
   code        points to start of the byte code
   utf         TRUE in UTF mode
   cb          compile data block
 
-Returns:      nothing
+Returns:      0 for success
+              -1 if a non-existant opcode is encountered
 */
 
-void
+int
 PRIV(auto_possessify)(PCRE2_UCHAR *code, BOOL utf, const compile_block *cb)
 {
 register PCRE2_UCHAR c;
@@ -1111,7 +1114,9 @@ uint32_t list[8];
 for (;;)
   {
   c = *code;
-
+  
+  if (c > OP_TABLE_LENGTH) return -1;   /* Something gone wrong */ 
+  
   if (c >= OP_STAR && c <= OP_TYPEPOSUPTO)
     {
     c -= get_repeat_base(c) - OP_STAR;
@@ -1207,7 +1212,7 @@ for (;;)
   switch(c)
     {
     case OP_END:
-    return;
+    return 0;
 
     case OP_TYPESTAR:
     case OP_TYPEMINSTAR:
diff --git a/src/pcre2_compile.c b/src/pcre2_compile.c
index 27089b1..d829e28 100644
--- a/src/pcre2_compile.c
+++ b/src/pcre2_compile.c
@@ -573,7 +573,7 @@ enum { ERR0 = COMPILE_ERROR_BASE,
        ERR41, ERR42, ERR43, ERR44, ERR45, ERR46, ERR47, ERR48, ERR49, ERR50,
        ERR51, ERR52, ERR53, ERR54, ERR55, ERR56, ERR57, ERR58, ERR59, ERR60,
        ERR61, ERR62, ERR63, ERR64, ERR65, ERR66, ERR67, ERR68, ERR69, ERR70,
-       ERR71, ERR72, ERR73, ERR74, ERR75, ERR76, ERR77, ERR78, ERR79 };
+       ERR71, ERR72, ERR73, ERR74, ERR75, ERR76, ERR77, ERR78, ERR79, ERR80 };
 
 /* This is a table of start-of-pattern options such as (*UTF) and settings such
 as (*LIMIT_MATCH=nnnn) and (*CRLF). For completeness and backward
@@ -7802,7 +7802,9 @@ if (usedlength > length) errorcode = ERR23; else
   }
 
 /* Fill in any forward references that are required. There may be repeated
-references; optimize for them, as searching a large regex takes time. */
+references; optimize for them, as searching a large regex takes time. The
+test of errorcode inside the loop means that nothing is done if it is already 
+non-zero. */
 
 if (cb.hwm > cb.start_workspace)
   {
@@ -7832,23 +7834,23 @@ if (cb.workspace_size > COMPILE_WORK_SIZE)
     ccontext->memctl.memory_data);
 cb.start_workspace = NULL;
 
-/* Give an error if there's back reference to a non-existent capturing
-subpattern. */
+/* After a successful compile, give an error if there's back reference to a
+non-existent capturing subpattern. Then, unless disabled, check whether any
+single character iterators can be auto-possessified. The function overwrites
+the appropriate opcode values, so the type of the pointer must be cast. NOTE:
+the intermediate variable "temp" is used in this code because at least one
+compiler gives a warning about loss of "const" attribute if the cast
+(PCRE2_UCHAR *)codestart is used directly in the function call. */
 
-if (errorcode == 0 && re->top_backref > re->top_bracket) errorcode = ERR15;
-
-/* Unless disabled, check whether any single character iterators can be
-auto-possessified. The function overwrites the appropriate opcode values, so
-the type of the pointer must be cast. NOTE: the intermediate variable "temp" is
-used in this code because at least one compiler gives a warning about loss of
-"const" attribute if the cast (PCRE2_UCHAR *)codestart is used directly in the
-function call. */
-
-if ((options & PCRE2_NO_AUTO_POSSESS) == 0)
+if (errorcode == 0)
   {
-  PCRE2_UCHAR *temp = (PCRE2_UCHAR *)codestart;
-  PRIV(auto_possessify)(temp, utf, &cb);
-  }
+  if (re->top_backref > re->top_bracket) errorcode = ERR15;
+  else if ((options & PCRE2_NO_AUTO_POSSESS) == 0)
+    {
+    PCRE2_UCHAR *temp = (PCRE2_UCHAR *)codestart;
+    if (PRIV(auto_possessify)(temp, utf, &cb) != 0) errorcode = ERR80;
+    }
+  }   
 
 /* If there were any lookbehind assertions that contained OP_RECURSE
 (recursions or subroutine calls), a flag is set for them to be checked here,
@@ -7858,7 +7860,7 @@ OP_RECURSE that are not fixed length get a diagnosic with a useful offset. The
 exceptional ones forgo this. We scan the pattern to check that they are fixed
 length, and set their lengths. */
 
-if (cb.check_lookbehind)
+if (errorcode == 0 && cb.check_lookbehind)
   {
   PCRE2_UCHAR *cc = (PCRE2_UCHAR *)codestart;
 
diff --git a/src/pcre2_error.c b/src/pcre2_error.c
index b257c60..3cd8792 100644
--- a/src/pcre2_error.c
+++ b/src/pcre2_error.c
@@ -159,6 +159,8 @@ static const char compile_error_texts[] =
   "character code point value in \\u.... sequence is too large\0"
   "digits missing in \\x{} or \\o{}\0"
   "syntax error in (?(VERSION condition\0"
+  /* 80 */ 
+  "internal error: unknown opcode in auto_possessify()\0"
   ;
 
 /* Match-time and UTF error texts are in the same format. */
diff --git a/src/pcre2_internal.h b/src/pcre2_internal.h
index ebc0197..ba8906a 100644
--- a/src/pcre2_internal.h
+++ b/src/pcre2_internal.h
@@ -1882,7 +1882,7 @@ is available. */
 #define _pcre2_was_newline           PCRE2_SUFFIX(_pcre2_was_newline_)
 #define _pcre2_xclass                PCRE2_SUFFIX(_pcre2_xclass_)
 
-extern void         _pcre2_auto_possessify(PCRE2_UCHAR *, BOOL,
+extern int          _pcre2_auto_possessify(PCRE2_UCHAR *, BOOL,
                       const compile_block *);
 extern PCRE2_SPTR   _pcre2_find_bracket(PCRE2_SPTR, BOOL, int);
 extern BOOL         _pcre2_is_newline(PCRE2_SPTR, uint32_t, PCRE2_SPTR,