diff --git a/src/calc.cpp b/src/calc.cpp
index 42712eeb1..d85c361ff 100644
--- a/src/calc.cpp
+++ b/src/calc.cpp
@@ -114,7 +114,7 @@ int main (int argc, char** argv)
       e.evaluatePostfixExpression (expression, result);
 
     // Show any debug output.
-    for (auto& i : context.debugMessages)
+    for (const auto& i : context.debugMessages)
       std::cout << i << '\n';
 
     // Show the result in string form.