How can I use reflection to convert from int to decimal?

I have some code (which works fine) that looks something like this:

        int integer = 42;
        decimal? castTo = integer;

Then I wanted to do something similar with reflection, with some code that looks like this:

object value = source; // source was an int originally
var parameters = new object[1];    
    ...
    parameters[0] = value;
    var setMethod = property.GetSetMethod();     
    // Call the set method, which takes a decimal? as a parameter
    setMethod.Invoke(o, parameters);  

When I do this, I get:

failed: System.ArgumentException : Object of type 'System.Int32' cannot be converted to type 'System.Nullable`1[System.Decimal]'.
    at System.RuntimeType.CheckValue(Object value, Binder binder, CultureInfo culture, BindingFlags invokeAttr)
    at System.Reflection.MethodBase.CheckArguments(Object[] parameters, Binder binder, BindingFlags invokeAttr, CultureInfo culture, Signature sig)
    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
    at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)

Why would an implicit type conversion that works fine elsewhere fail with reflection? Is there a trick to using reflection to perform this conversion?

Edit: Thanks all for the responses. Here is the solution I came up with, based on the answers:

    private object Convert(object source, Type destinationType)
    {
        if (source == null)
        {
            return null;
        }

        var sourceType = source.GetType();

        // unwrap nullable types
        var nullableType = Nullable.GetUnderlyingType(destinationType);