Mapping tool - how to add labels to isolines

I created a color plot with the mapping tool, using pcolorm. Easy. I added isolines at certain values, using contourm. Still no problem. Now I want to add labels (like 'low', 'high', 'dangerous', etc) to each line. That doesn't work somehow. I tried the same command I used in 'standard plots' (no mapping tool), e.g. set(findobj(th,'String',strtlines(1,:)),'String','LOW'); but it seems Matlab with mapping tool ignores this. Replacing set by setm results in a cryptic error message about inconsistency of operands in a logical operation.

Any tip how to solve is appreciated.

Thanks in advance, Peter