How to display dialog "error" when variable returns incorrect answer

Question

Okay so my code is below and when I input a word spelled incorrectly or a phrase that doesn't make sense it displays a dialog with "msng" but I want I to display an error message instead. I tried using both if theAnswer is/ contains "msng" then... but it will not work, any help is appreciated.

tell application "Safari"
    quit
end tell

set defaultAnswer to ""
set cancelButton to "Cancel"
set buttonResearch to "ReSearch"

display dialog "Query: " default answer defaultAnswer buttons {cancelButton, buttonResearch} default button buttonResearch cancel button cancelButton with icon 1
copy the result as list to {button_pressed, text_returned}

tell application "Dragon Dictate"
    set listening to false
end tell

if (button_pressed is buttonResearch) and (text_returned is not "") then
    set theUrl to "http://www.wolframalpha.com/input/?i=" & encode_text(text_returned, true, false)
    tell application "Safari"
        tell window 1 to set current tab to (make new tab with properties {URL:theUrl})
        tell me to say "let me look that up for you now"
        tell document 1
            repeat -- wait until loaded
                delay 2
                if (do JavaScript "document.readyState") = "complete" then exit repeat
            end repeat
            do JavaScript "document.getElementById('pod_0200').getElementsByClassName('action subpod-copyablept ')[0].click()" -- show the popup window
            set theAnswer to do JavaScript "document.body.lastChild.getElementsByTagName('pre')[0].innerHTML;" -- get the answer in this popup window
        end tell
    end tell
    activate

    if theAnswer contains "msng" then
        display dialog "There was an error, you may have misspelled a word or phrased it incorrectly"
    else
        display dialog theAnswer
    end if

end if

tell application "Safari"
    quit
end tell

-- encoding high-ASCII characters:
on encode_char(this_char)
    set the ASCII_num to (the ASCII number this_char)
    set the hex_list to {"0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "A", "B", "C", "D", "E", "F"}
    set x to item ((ASCII_num div 16) + 1) of the hex_list
    set y to item ((ASCII_num mod 16) + 1) of the hex_list
    return ("%" & x & y) as string
end encode_char

-- TEXT ENCODING: encode spaces and high-level ASCII characters (those above 127)
-- encode_URL_A = encode most of the special characters reserved for use by URLs.
on encode_text(this_text, encode_URL_A, encode_URL_B)
    set the standard_characters to "abcdefghijklmnopqrstuvwxyz0123456789"
    set the URL_A_chars to "$+!'/?;&@=#%><{}[]\"~`^\\|*"
    set the URL_B_chars to ".-_:"
    set the acceptable_characters to the standard_characters
    if encode_URL_A is false then set the acceptable_characters to the acceptable_characters & the URL_A_chars
    if encode_URL_B is false then set the acceptable_characters to the acceptable_characters & the URL_B_chars
    set the encoded_text to ""
    repeat with this_char in this_text
        if this_char is in the acceptable_characters then
            set the encoded_text to (the encoded_text & this_char)
        else
            set the encoded_text to (the encoded_text & encode_char(this_char)) as string
        end if
    end repeat
    return the encoded_text
end encode_text

Answer 1

Use

if theAnswer is missing value then
    display dialog "There was an error, you may have misspelled a word or phrased it incorrectly"
else
    display dialog theAnswer
end if

Because «class msng» is the raw AppleScript code for missing value.

The display dialog command show the name of this class --> "msng"