Has anyone come across exceeding Key byte limits in a map/reduce?

{"type":"error.SuiteScriptError","name":"JS_EXCEPTION","message":"org.mozilla.javascript.WrappedException: Wrapped java.lang.RuntimeException: Map Reduce Task [ 955 ] Stage [ REDUCE ]: Key is over 500 bytes! (INVOCATION_WRAPPER$sys#77)"}

Documentation says I should have 4000 bytes, not 500