In objective-c how do I use object_setIvar to set a primitive value?

I have this:

Ivar ivar = class_getInstanceVariable([self class], [name UTF8String]);
CGFloat floatVal = [coder decodeFloatForKey:name];

I'd then like to do this:

object_setIvar(self, ivar, floatVal);

...but I can't just use floatVal like this - what is the syntax for using floatVal in this context?