Error "QString::arg: Argument missing"

[Lawand]

When building and executing the following code:

#include <QString>
#include <iostream>
 
int main()
{
	QString someString = QString("1%").arg("Hello World!");
 
    std::cout << someString.toStdString();
 
    return 0;
}

#include <QString>
#include <iostream>

int main()
{
	QString someString = QString("1%").arg("Hello World!");

    std::cout << someString.toStdString();

    return 0;
}

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I get this output:

QString::arg: Argument missing: 1%, Hello World!
1%

QString::arg: Argument missing: 1%, Hello World!
1%

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which includes the error message "QString::arg: Argument missing", why am I getting this??

Thanks in advance.