diff --git a/lib/checkio.cpp b/lib/checkio.cpp
index 4d881457e..910c12445 100644
--- a/lib/checkio.cpp
+++ b/lib/checkio.cpp
@@ -176,7 +176,7 @@ void CheckIO::checkFileUsage()
                     operation = Filepointer::OPEN;
                 } else if ((tok->str() == "rewind" || tok->str() == "fseek" || tok->str() == "fsetpos" || tok->str() == "fflush") ||
                            (windows && tok->str() == "_fseeki64")) {
-                    if (Token::simpleMatch(tok, "fflush ( stdin )"))
+                    if (_settings->isEnabled("portability") && Token::simpleMatch(tok, "fflush ( stdin )"))
                         fflushOnInputStreamError(tok, tok->strAt(2));
                     else {
                         fileTok = tok->tokAt(2);
@@ -297,8 +297,8 @@ void CheckIO::checkFileUsage()
 
 void CheckIO::fflushOnInputStreamError(const Token *tok, const std::string &varname)
 {
-    reportError(tok, Severity::error,
-                "fflushOnInputStream", "fflush() called on input stream '" + varname + "' results in undefined behaviour.");
+    reportError(tok, Severity::portability,
+                "fflushOnInputStream", "fflush() called on input stream '" + varname + "' may result in undefined behaviour on non-linux systems.");
 }
 
 void CheckIO::ioWithoutPositioningError(const Token *tok)
diff --git a/test/testio.cpp b/test/testio.cpp
index 9348bff41..a597e2cb5 100644
--- a/test/testio.cpp
+++ b/test/testio.cpp
@@ -581,13 +581,13 @@ private:
         check("void foo()\n"
               "{\n"
               "    fflush(stdin);\n"
-              "}");
-        ASSERT_EQUALS("[test.cpp:3]: (error) fflush() called on input stream 'stdin' results in undefined behaviour.\n", errout.str());
+              "}", false, true);
+        ASSERT_EQUALS("[test.cpp:3]: (portability) fflush() called on input stream 'stdin' may result in undefined behaviour on non-linux systems.\n", errout.str());
 
         check("void foo()\n"
               "{\n"
               "    fflush(stdout);\n"
-              "}");
+              "}", false, true);
         ASSERT_EQUALS("", errout.str());
     }