diff --git a/lib/checkcondition.h b/lib/checkcondition.h
index 9b5300260..cc9e7805a 100644
--- a/lib/checkcondition.h
+++ b/lib/checkcondition.h
@@ -48,6 +48,7 @@ public:
         CheckCondition checkCondition(tokenizer, settings, errorLogger);
         checkCondition.multiCondition();
         checkCondition.clarifyCondition();   // not simplified because ifAssign
+        checkCondition.oppositeInnerCondition();
     }
 
     /** @brief Run checks against the simplified token list */
@@ -56,7 +57,6 @@ public:
         checkCondition.assignIf();
         checkCondition.checkBadBitmaskCheck();
         checkCondition.comparison();
-        checkCondition.oppositeInnerCondition();
         checkCondition.checkIncorrectLogicOperator();
         checkCondition.checkModuloAlwaysTrueFalse();
         checkCondition.alwaysTrueFalse();
diff --git a/test/testcondition.cpp b/test/testcondition.cpp
index 1311b7814..81c7956e6 100644
--- a/test/testcondition.cpp
+++ b/test/testcondition.cpp
@@ -1309,6 +1309,23 @@ private:
               "  }\n"
               "}");
         ASSERT_EQUALS("", errout.str());
+
+        // #6227 - FP caused by simplifications of casts and known variables
+        check("void foo(A *a) {\n"
+              "   if(a) {\n"
+              "       B *b = dynamic_cast<B*>(a);\n"
+              "       if(!b) {}\n"
+              "    }\n"
+              "}");
+        ASSERT_EQUALS("", errout.str());
+
+        check("void foo(int a) {\n"
+              "   if(a) {\n"
+              "       int b = a;\n"
+              "       if(!b) {}\n"
+              "    }\n"
+              "}");
+        TODO_ASSERT_EQUALS("error", "", errout.str());
     }
 
     // clarify conditions with = and comparison