What is this error message telling me?

[Anonymous via Digitalmars-d-learn]

I was watching a dconf presentation from last year and wanted to 
try this out: https://github.com/luismarques/parnas72. It doesn't 
compile / run as it is and the problem seems to be in the 
function below.

import std.algorithm;
import std.range;
import std.uni;
/// Performs [["foo", "bar"], ["baz"]] -> ["baz", "foo bar"]
auto alphabetized(Range)(Range range)
{
     return range
         .map!(line => line.joiner(" "))
         .array
         .sort!((a, b) => icmp(a, b) < 0);
}

void main()
{
	auto a = alphabetized([["foo", "bar"], ["baz"]]);
}


More specifically, icmp doesn't seem to be allowed as the 
predicate for sort:

Here's the error message I get:

C:\D\dmd2\windows\bin\..\..\src\phobos\std\uni.d(7082): Error: 
function 'std.algorithm.searching.skipOver!(Result, 
dstring).skipOver' is not nothrow
C:\D\dmd2\windows\bin\..\..\src\phobos\std\uni.d(7055): Error: 
nothrow function 'std.uni.fullCasedCmp!(Result).fullCasedCmp' may 
throw
C:\D\dmd2\windows\bin\..\..\src\phobos\std\uni.d(7136): Error: 
template instance std.uni.fullCasedCmp!(Result) error 
instantiating
test.d(14):        instantiated from here: icmp!(Result, Result)
C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm\sorting.d(1851):        instantiated from here: __lambda3!(Result, Result)
test.d(14):        instantiated from here: sort!((a, b) => 
icmp(a, b) < 0, cast(SwapStrategy)0, Result[])
test.d(19):        instantiated from here: 
alphabetized!(string[][])
C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm\sorting.d(1863): Error: static assert  "Invalid predicate passed to sort: __lambda3"
test.d(14):        instantiated from here: sort!((a, b) => 
icmp(a, b) < 0, cast(SwapStrategy)0, Result[])
test.d(19):        instantiated from here: 
alphabetized!(string[][])


My question is, how do I begin to understand error messages like 
the above? I looked at the signature for sort and icmp and don't 
get what the problem is.

Thanks.