diff --git a/lib/core/settings.py b/lib/core/settings.py
index 70e9340ea..08cf6c1a3 100644
--- a/lib/core/settings.py
+++ b/lib/core/settings.py
@@ -19,7 +19,7 @@ from lib.core.enums import OS
 from lib.core.revision import getRevisionNumber
 
 # sqlmap version (<major>.<minor>.<month>.<monthly commit>)
-VERSION = "1.0.9.20"
+VERSION = "1.0.9.21"
 REVISION = getRevisionNumber()
 TYPE = "dev" if VERSION.count('.') > 2 and VERSION.split('.')[-1] != '0' else "stable"
 TYPE_COLORS = {"dev": 33, "stable": 90, "pip": 34}
diff --git a/lib/techniques/blind/inference.py b/lib/techniques/blind/inference.py
index 4b02587f7..63806d027 100644
--- a/lib/techniques/blind/inference.py
+++ b/lib/techniques/blind/inference.py
@@ -144,7 +144,7 @@ def bisection(payload, expression, length=None, charsetType=None, firstChar=None
             length = None
 
         showEta = conf.eta and isinstance(length, int)
-        numThreads = min(conf.threads, length)
+        numThreads = min(conf.threads, length) or 1
 
         if showEta:
             progress = ProgressBar(maxValue=length)
@@ -279,7 +279,7 @@ def bisection(payload, expression, length=None, charsetType=None, firstChar=None
                         finally:
                             firstCheck = True
 
-                    elif not lastCheck:
+                    elif not lastCheck and numThreads == 1:  # not usable in multi-threading environment
                         if charTbl[(len(charTbl) >> 1)] < ord(' '):
                             try:
                                 # favorize last char check if current value inclines toward 0
diff --git a/txt/checksum.md5 b/txt/checksum.md5
index 204cca0f7..43f049386 100644
--- a/txt/checksum.md5
+++ b/txt/checksum.md5
@@ -45,7 +45,7 @@ e60456db5380840a586654344003d4e6  lib/core/readlineng.py
 5ef56abb8671c2ca6ceecb208258e360  lib/core/replication.py
 99a2b496b9d5b546b335653ca801153f  lib/core/revision.py
 7c15dd2777af4dac2c89cab6df17462e  lib/core/session.py
-95276012feeed6d7d2dc6f22724c70cc  lib/core/settings.py
+64150b622fb8fc638de6aa98bcee6ff5  lib/core/settings.py
 7af83e4f18cab6dff5e67840eb65be80  lib/core/shell.py
 23657cd7d924e3c6d225719865855827  lib/core/subprocessng.py
 0bc2fae1dec18cdd11954b22358293f2  lib/core/target.py
@@ -87,7 +87,7 @@ cc9c82cfffd8ee9b25ba3af6284f057e  lib/takeover/__init__.py
 7d6cd7bdfc8f4bc4e8aed60c84cdf87f  lib/takeover/udf.py
 f6e3084abd506925a8be3d1c0a6d058c  lib/takeover/web.py
 9af83a62de360184f1c14e69b8a95cfe  lib/takeover/xp_cmdshell.py
-f9fccc94cb9d5c15f84b5feb579ab0de  lib/techniques/blind/inference.py
+03b8c2028e996c471e45308fa518d79a  lib/techniques/blind/inference.py
 cc9c82cfffd8ee9b25ba3af6284f057e  lib/techniques/blind/__init__.py
 cc9c82cfffd8ee9b25ba3af6284f057e  lib/techniques/brute/__init__.py
 d36effffe64e63ef9b3be490f850e2cc  lib/techniques/brute/use.py