"Block pointer to non-function type is invalid"

Here I have a method to encode a string (it is incomplete), and you will find my problem is an error: "Block pointer to non-function type is invalid"

+ (NSString *)encodeString: (NSString *)string {
    __block int indexShift;
    __block NSString *dictionary = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    __block NSString *encodeDictionary = @"mahgbcjdfukripylswxovzetqnFMAJWGCQYXLOETPBKSVNIZUHDR";
    __block NSString *encodeString = @"";

    void (^encode) = ^{ // Error here, "Block pointer to non-function type is invalid"
        for (int x = 0; x < string.length; x++) {
            int index = [dictionary indexOf:[string characterAtIndex:x]];
            indexShift += index;
            encodeString = [encodeString stringByAppendingFormat:@"%c", [encodeDictionary characterAtIndex:index+indexShift]];
        }
    };

    return encodeString;
}

Please tell me why this is happening, or what I need to change to fix it.