diff --git a/src/tools/moc/preprocessor.cpp b/src/tools/moc/preprocessor.cpp
index adba0f9b9b..d036c40f35 100644
--- a/src/tools/moc/preprocessor.cpp
+++ b/src/tools/moc/preprocessor.cpp
@@ -529,7 +529,7 @@ Symbols Preprocessor::tokenize(const QByteArray& input, int lineNum, Preprocesso
     return symbols;
 }
 
-void Preprocessor::macroExpand(Symbols *into, Preprocessor *that, Symbols &toExpand, int &index,
+void Preprocessor::macroExpand(Symbols *into, Preprocessor *that, const Symbols &toExpand, int &index,
                                   int lineNum, bool one, const QSet<QByteArray> &excludeSymbols)
 {
     SymbolStack symbols;
diff --git a/src/tools/moc/preprocessor.h b/src/tools/moc/preprocessor.h
index 8d3bf4edbe..9c81f86f9c 100644
--- a/src/tools/moc/preprocessor.h
+++ b/src/tools/moc/preprocessor.h
@@ -76,7 +76,7 @@ public:
 
     void substituteUntilNewline(Symbols &substituted);
     static Symbols macroExpandIdentifier(Preprocessor *that, SymbolStack &symbols, int lineNum, QByteArray *macroName);
-    static void macroExpand(Symbols *into, Preprocessor *that, Symbols &toExpand, int &index, int lineNum, bool one,
+    static void macroExpand(Symbols *into, Preprocessor *that, const Symbols &toExpand, int &index, int lineNum, bool one,
                                const QSet<QByteArray> &excludeSymbols = QSet<QByteArray>());
 
     int evaluateCondition();