diff --git a/ChangeLog b/ChangeLog
index 6279995..e6afed9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -62,6 +62,9 @@ first of these bugs was discovered by Karl Skomski with the LLVM fuzzer.
 pcre2_compile() to run for a very long time. This bug was found by the LLVM 
 fuzzer.
 
+17. A missing closing parenthesis for a callout with a string argument was not
+being diagnosed, possibly leading to a buffer overflow. This bug was found by
+the LLVM fuzzer.
 
 
 Version 10.20 30-June-2015
diff --git a/src/pcre2_compile.c b/src/pcre2_compile.c
index b9f7e7f..940f693 100644
--- a/src/pcre2_compile.c
+++ b/src/pcre2_compile.c
@@ -3277,46 +3277,51 @@ for (; ptr < cb->end_pattern; ptr++)
       if (IS_DIGIT(ptr[1]))
         {
         while (IS_DIGIT(ptr[1])) ptr++;
-        if (ptr[1] != CHAR_RIGHT_PARENTHESIS)
-          {
-          errorcode = ERR39;
-          ptr++;
-          goto FAILED;
-          }
-        break;
         }
 
       /* Handle a string argument */
 
-      ptr++;
-      delimiter = 0;
-      for (i = 0; PRIV(callout_start_delims)[i] != 0; i++)
-        {
-        if (*ptr == PRIV(callout_start_delims)[i])
+      else
+        { 
+        ptr++;
+        delimiter = 0;
+        for (i = 0; PRIV(callout_start_delims)[i] != 0; i++)
           {
-          delimiter = PRIV(callout_end_delims)[i];
-          break;
+          if (*ptr == PRIV(callout_start_delims)[i])
+            {
+            delimiter = PRIV(callout_end_delims)[i];
+            break;
+            }
           }
-        }
-
-      if (delimiter == 0)
-        {
-        errorcode = ERR82;
-        goto FAILED;
-        }
-
-      start = ptr;
-      do
-        {
-        if (++ptr >= cb->end_pattern)
+        
+        if (delimiter == 0)
           {
-          errorcode = ERR81;
-          ptr = start;   /* To give a more useful message */
+          errorcode = ERR82;
           goto FAILED;
           }
-        if (ptr[0] == delimiter && ptr[1] == delimiter) ptr += 2;
+        
+        start = ptr;
+        do
+          {
+          if (++ptr >= cb->end_pattern)
+            {
+            errorcode = ERR81;
+            ptr = start;   /* To give a more useful message */
+            goto FAILED;
+            }
+          if (ptr[0] == delimiter && ptr[1] == delimiter) ptr += 2;
+          }
+        while (ptr[0] != delimiter);
+        } 
+        
+      /* Check terminating ) */
+
+      if (ptr[1] != CHAR_RIGHT_PARENTHESIS)
+        {
+        errorcode = ERR39;
+        ptr++;
+        goto FAILED;
         }
-      while (ptr[0] != delimiter);
       break;
 
       case CHAR_LEFT_PARENTHESIS:
diff --git a/testdata/testinput5 b/testdata/testinput5
index 7e2ba45..47afd25 100644
--- a/testdata/testinput5
+++ b/testdata/testinput5
@@ -1651,4 +1651,6 @@
 
 /$(&.+[\p{Me}].\s\xdcC*?(?(<y>))(?<!^)$C((;*?(R))+(?(R)){0,6}?|){12\x8a\X*?\x8a\x0b\xd1^9\3*+(\xc1,\k'P'\xb4)\xcc(z\z(?JJ)(?'X'8};(\x0b\xd1^9\?'3*+(\xc1.]k+\x0b'Pm'\xb4\xcc4'\xd1'(?'X'))?-%--\x95$9*\4'|\xd1(''%\x95*$9)#(?'R')3\x07?('P\xed')\\x16:;()\x1e\x10*:(?<y>)\xd1+!~:(?)''(d'E:yD!\s(?'R'\x1e;\x10:U))|')g!\xb0*){29+))#(?'P'})*?/
 
+"(*UTF)(*UCP)(.UTF).+X(\V+;\^(\D|)!999}(?(?C{7(?C')\H*\S*/^\x5\xa\\xd3\x85n?(;\D*(?m).[^mH+((*UCP)(*U:F)})(?!^)(?'"
+
 # End of testinput5 
diff --git a/testdata/testoutput5 b/testdata/testoutput5
index a99c12b..641c563 100644
--- a/testdata/testoutput5
+++ b/testdata/testoutput5
@@ -4050,4 +4050,7 @@ Failed: error 122 at offset 1227: unmatched closing parenthesis
 
 /$(&.+[\p{Me}].\s\xdcC*?(?(<y>))(?<!^)$C((;*?(R))+(?(R)){0,6}?|){12\x8a\X*?\x8a\x0b\xd1^9\3*+(\xc1,\k'P'\xb4)\xcc(z\z(?JJ)(?'X'8};(\x0b\xd1^9\?'3*+(\xc1.]k+\x0b'Pm'\xb4\xcc4'\xd1'(?'X'))?-%--\x95$9*\4'|\xd1(''%\x95*$9)#(?'R')3\x07?('P\xed')\\x16:;()\x1e\x10*:(?<y>)\xd1+!~:(?)''(d'E:yD!\s(?'R'\x1e;\x10:U))|')g!\xb0*){29+))#(?'P'})*?/
 
+"(*UTF)(*UCP)(.UTF).+X(\V+;\^(\D|)!999}(?(?C{7(?C')\H*\S*/^\x5\xa\\xd3\x85n?(;\D*(?m).[^mH+((*UCP)(*U:F)})(?!^)(?'"
+Failed: error 139 at offset 113: closing parenthesis for (?C expected
+
 # End of testinput5