diff --git a/lib/valueflow.cpp b/lib/valueflow.cpp
index e8a1b5f77..eba7d7c52 100644
--- a/lib/valueflow.cpp
+++ b/lib/valueflow.cpp
@@ -726,6 +726,13 @@ static bool valueFlowForward(Token * const               startToken,
 
         // conditional block of code that assigns variable..
         else if (Token::Match(tok2, "%var% (") && Token::simpleMatch(tok2->linkAt(1), ") {")) {
+            // is variable changed in condition?
+            if (isVariableChanged(tok2->next(), tok2->next()->link(), varid)) {
+                if (settings->debugwarnings)
+                    bailout(tokenlist, errorLogger, tok2, "variable " + var->name() + " valueFlowForward, assignment in condition");
+                return false;
+            }
+
             // Should scope be skipped because variable value is checked?
             std::list<ValueFlow::Value> truevalues;
             for (std::list<ValueFlow::Value>::iterator it = values.begin(); it != values.end(); ++it) {
diff --git a/test/testvalueflow.cpp b/test/testvalueflow.cpp
index 6b02f79a2..6ffdc052c 100644
--- a/test/testvalueflow.cpp
+++ b/test/testvalueflow.cpp
@@ -873,6 +873,14 @@ private:
         ASSERT_EQUALS(false, testValueOfX(code, 8U, 34));
 
         // while/for
+        code = "void f() {\n" // #6138
+               "  ENTRY *x = 0;\n"
+               "  while (x = get()) {\n"
+               "    set(x->value);\n"  // <- x is not 0
+               "  }\n"
+               "}\n";
+        ASSERT_EQUALS(false, testValueOfX(code, 4U, 0));
+
         code = "void f(const int *buf) {\n"
                "  int x = 0;\n"
                "  for (int i = 0; i < 10; i++) {\n"
@@ -1049,7 +1057,7 @@ private:
                "    for (; x && \n"
                "         x->str() != y; x = x->next()) {}\n"
                "}";
-        ASSERT_EQUALS(true, testValueOfX(code, 3U, 0));
+        TODO_ASSERT_EQUALS(true, false, testValueOfX(code, 3U, 0));
         ASSERT_EQUALS(false, testValueOfX(code, 4U, 0));
 
         code = "void f(const Token* x) {\n"