             XCTAssertEqual(i * 1048576, BitUnit.convert(i, from: .Pebibyte, to: .Gibibyte))
             XCTAssertEqual(i * 1024, BitUnit.convert(i, from: .Pebibyte, to: .Tebibyte))
         }
+        XCTAssertNil(BitUnit.convert(-1, from: .Pebibyte, to: .Tebibyte))
     }
     
     func testStringFormatting() {

            XCTAssertEqual(i * 1000000, BitUnit.convert(i, from: .Petabit, to: .Gigabit))
            XCTAssertEqual(i * 1000, BitUnit.convert(i, from: .Petabit, to: .Terabit))
        }
    }
    
    func testBinaryConversions() {

        
        XCTAssertEqual("2.9 Mib", BitUnit.format(3, sourceUnit: .Megabit, targetUnitType: .BinaryBitUnit, formatter: formatter))
        
        XCTAssertNil(BitUnit.format(-1, sourceUnit: .Megabit, targetUnitType: .BinaryBitUnit, formatter: formatter))
    }
    
    func testAbbreviations() {
