diff --git a/lib/core/agent.py b/lib/core/agent.py
index 2df03da44..b8abc5567 100644
--- a/lib/core/agent.py
+++ b/lib/core/agent.py
@@ -32,6 +32,8 @@ from lib.core.enums import PLACE
 from lib.core.enums import POST_HINT
 from lib.core.exception import SqlmapNoneDataException
 from lib.core.settings import CUSTOM_INJECTION_MARK_CHAR
+from lib.core.settings import DEFAULT_COOKIE_DELIMITER
+from lib.core.settings import DEFAULT_GET_POST_DELIMITER
 from lib.core.settings import GENERIC_SQL_COMMENT
 from lib.core.settings import PAYLOAD_DELIMITER
 from lib.core.settings import REPLACEMENT_MARKER
@@ -156,7 +158,10 @@ class Agent(object):
         elif place in (PLACE.USER_AGENT, PLACE.REFERER, PLACE.HOST):
             retVal = paramString.replace(origValue, self.addPayloadDelimiters(newValue))
         else:
-            retVal = re.sub(r"(\A|\b)%s=%s" % (re.escape(parameter), re.escape(origValue)), "%s=%s" % (parameter, self.addPayloadDelimiters(newValue.replace("\\", "\\\\"))), paramString)
+            if origValue:
+                retVal = re.sub(r"(\A|\b)%s=%s(\Z|\b)" % (re.escape(parameter), re.escape(origValue)), "%s=%s" % (parameter, self.addPayloadDelimiters(newValue.replace("\\", "\\\\"))), paramString)
+            else:
+                retVal = re.sub(r"(\A|\b)%s=%s(\Z|%s|%s|\s)" % (re.escape(parameter), re.escape(origValue), DEFAULT_GET_POST_DELIMITER, DEFAULT_COOKIE_DELIMITER), "%s=%s\g<2>" % (parameter, self.addPayloadDelimiters(newValue.replace("\\", "\\\\"))), paramString)
             if retVal == paramString and urlencode(parameter) != parameter:
                 retVal = re.sub(r"(\A|\b)%s=%s" % (re.escape(urlencode(parameter)), re.escape(origValue)), "%s=%s" % (urlencode(parameter), self.addPayloadDelimiters(newValue.replace("\\", "\\\\"))), paramString)