Passing A Map As Parameter

I'm using Java in BlueJ where I'm trying to write a class to store cycling club membership details in a map. I'm trying to pass a map as a parameter to a function that will iterate over the map and print out the member details. My code is:

public class CtcMembers
{
//instance variables
private Map<String, Set<String>> memberMap;
/**
* Constructor for objects of class CtcMembers

[Code] ....

When I execute the following in the OU workspace:

CtcMembers c = new CtcMembers();
c.addCyclists();

I can see a map populated with the expected details.

When I execute the following in the OU workspace:

c.printMap(c.getMap());

I get the following error:

java.lang.ClassCastException: java.util.HashSet cannot be cast to java.lang.String
in CtcMembers.printMap(CtcMembers.java:81)
in (OUWorkspace:1)

I wasn't aware I was trying to cast anything so this has got me baffled.