diff --git a/lib/request/inject.py b/lib/request/inject.py
index 6448cdbf9..322e3cd9b 100644
--- a/lib/request/inject.py
+++ b/lib/request/inject.py
@@ -29,6 +29,7 @@ from lib.core.common import pushValue
 from lib.core.common import randomInt
 from lib.core.common import readInput
 from lib.core.common import safeStringFormat
+from lib.core.common import singleTimeWarnMessage
 from lib.core.data import conf
 from lib.core.data import kb
 from lib.core.data import logger
@@ -446,6 +447,12 @@ def getValue(expression, blind=True, inband=True, error=True, time=True, fromUse
 
     kb.safeCharEncode = False
 
+    if not kb.testMode and value is None:
+        warnMsg = "in case of continuous data retrieval problems you are advised to try "
+        warnMsg += "a hidden switch '--no-cast' (fixing problems with some collation "
+        warnMsg += "issues) and/or switch '--hex'"
+        singleTimeWarnMessage(warnMsg)
+
     return extractExpectedValue(value, expected)
 
 def goStacked(expression, silent=False):
diff --git a/lib/techniques/union/use.py b/lib/techniques/union/use.py
index c246ddeaf..8d7010815 100644
--- a/lib/techniques/union/use.py
+++ b/lib/techniques/union/use.py
@@ -94,11 +94,6 @@ def __oneShotUnionUse(expression, unpack=True, limited=False):
                 warnMsg = "possible server trimmed output detected (due to its length): "
                 warnMsg += trimmed
                 logger.warn(warnMsg)
-            elif Backend.isDbms(DBMS.MYSQL) and not kb.multiThreadMode:
-                warnMsg = "if the problem persists with 'None' values please try to use "
-                warnMsg += "hidden switch '--no-cast' (fixing problems with some collation "
-                warnMsg += "issues) or switch '--hex'"
-                singleTimeWarnMessage(warnMsg)
 
         hashDBWrite("%s%s" % (conf.hexConvert, expression), retVal)