Odd error complaint about overloaded method signatures when args have default values

For brevity, I'll just give the class and method signatures:
class TextMark { /** Insert a new substring before or after this mark. */
def insert(str: String, before: Boolean = true)
/** Insert a new mark adjacent to this one and return the new mark. */ def insert(mark: TextMark, before: Boolean = true): TextMark = {...}}
When I try to run a test, I get a compile error:
[error] /Users/Ken/Documents/Programming/Scala/contextual/src/main/scala/com/digitaldoodles/contextual/model/Text.scala:70: in class TextMark, multiple overloaded alternatives of method insert define default arguments.[error] class TextMark {
I could understand the problem if the first args had the same type and it was the second args that had differing types, because then the compiler would have no way of knowing which method to call if the optional second arg wasn't provided. But in this case, an invocation of "insert" will always be type-unambiguous. Am I missing something here?
Thanks,Ken