diff --git a/src/UniqueFileGenerator.Console/Arguments/ArgumentTypes.fs b/src/UniqueFileGenerator.Console/Arguments/ArgumentTypes.fs
index af720fc..4ee08ef 100644
--- a/src/UniqueFileGenerator.Console/Arguments/ArgumentTypes.fs
+++ b/src/UniqueFileGenerator.Console/Arguments/ArgumentTypes.fs
@@ -3,6 +3,7 @@ namespace UniqueFileGenerator.Console
 open Errors
 open Utilities
 open System
+open System.IO
 open FSharpPlus
 open CCFSharpUtils.Text
 
@@ -18,6 +19,15 @@ module ArgTypes =
         |> tryParseInRange (floor, ceiling)
         |> Result.mapError (fun _ -> NumberParseFailure (text, (floor, ceiling)))
 
+    /// Determines whether a string contains characters invalid for filnames on this OS.
+    let validateChars text =
+        let invalidChars = Path.GetInvalidFileNameChars() |> Set.ofArray
+        let isInvalidChar ch = Set.contains ch invalidChars
+        let rejectedChars = String.filter isInvalidChar text
+        if String.hasNoText rejectedChars
+            then Ok text
+            else Error (rejectedChars |> String.toList |> InvalidChars)
+
     type FileCount = private FileCount of int with
         static member val AllowedRange = 1, Int32.MaxValue
 
@@ -36,8 +46,8 @@ module ArgTypes =
 
         static member Create maybeText =
             maybeText
-            |> option id Prefix.Default
-            |> Prefix
+            |> option validateChars (Ok Prefix.Default)
+            |> map Prefix
 
         member this.Value = let (Prefix prefix) = this in prefix
 
@@ -50,7 +60,7 @@ module ArgTypes =
             |> option
                 (stripSeparatorsAndTrim >> tryParseInRange NameBaseLength.AllowedRange)
                 (Ok NameBaseLength.Default)
-            |> Result.map NameBaseLength
+            |> map NameBaseLength
 
         member this.Value = let (NameBaseLength length) = this in length
 
@@ -59,8 +69,8 @@ module ArgTypes =
 
         static member Create maybeText =
             maybeText
-            |> option String.trim Extension.Default
-            |> Extension
+            |> option (validateChars >> map String.trim) (Ok Extension.Default)
+            |> map Extension
 
         member this.Value = let (Extension ext) = this in ext
 
diff --git a/src/UniqueFileGenerator.Console/Arguments/ArgumentValidation.fs b/src/UniqueFileGenerator.Console/Arguments/ArgumentValidation.fs
index 7a7cd9b..64fa284 100644
--- a/src/UniqueFileGenerator.Console/Arguments/ArgumentValidation.fs
+++ b/src/UniqueFileGenerator.Console/Arguments/ArgumentValidation.fs
@@ -55,9 +55,9 @@ module ArgValidation =
             let! optionMap = toPairs (Array.tail args)
 
             let tryGetArg x = Map.tryFind flags[x] optionMap
-            let  p = Prefix.Create (tryGetArg Prefix)
+            let! p = Prefix.Create (tryGetArg Prefix)
             let! b = NameBaseLength.TryCreate (tryGetArg NameBaseLength)
-            let  e = Extension.Create (tryGetArg Extension)
+            let! e = Extension.Create (tryGetArg Extension)
             let  o = OutputDirectory.Create (tryGetArg OutputDirectory)
             let! s = Size.TryCreate (tryGetArg Size)
             let! d = Delay.TryCreate (tryGetArg Delay)
diff --git a/src/UniqueFileGenerator.Console/Errors.fs b/src/UniqueFileGenerator.Console/Errors.fs
index fcdb43a..01ca52a 100644
--- a/src/UniqueFileGenerator.Console/Errors.fs
+++ b/src/UniqueFileGenerator.Console/Errors.fs
@@ -1,6 +1,8 @@
 namespace UniqueFileGenerator.Console
 
 open Utilities
+open CCFSharpUtils.Text
+open FSharpPlus
 
 module Errors =
 
@@ -11,6 +13,7 @@ module Errors =
         | UnknownFlags
         | DuplicateFlags
         | NumberParseFailure of Input: string * AllowedRange: (int * int)
+        | InvalidChars of Chars: char list
         | DirectoryMissing of string
         | DriveSpaceConfirmationFailure
         | DriveSpaceInsufficient of Needed: string * Actual: string
@@ -24,7 +27,12 @@ module Errors =
         | UnknownFlags -> "Unknown option flag(s) found."
         | DuplicateFlags -> "Duplicate option flag(s) found. Each can only be used once."
         | NumberParseFailure (input, (floor, ceiling)) ->
-            $"The number \"%s{input}\" is out of bounds. Enter an integer between %s{Num.Format floor} and %s{Num.Format ceiling}, inclusive."
+            sprintf "\"%s\" is an invalid value. Enter an integer between %s and %s, inclusive."
+                input (Num.Format floor) (Num.Format ceiling)
+        | InvalidChars chars ->
+            sprintf "Found %s invalid for file names on this system: \"%s\"."
+                (String.pluralizeSWithCount "character" chars.Length) // "1 character," "2 characters," etc.
+                (chars |> map string |> String.concat "\", \"")
         | DirectoryMissing dirName -> $"Directory \"%s{dirName}\" was not found."
         | DriveSpaceConfirmationFailure -> "Could not confirm available drive space."
         | DriveSpaceInsufficient (needed, actual) ->
diff --git a/src/UniqueFileGenerator.Tests/ArgParserTests.fs b/src/UniqueFileGenerator.Tests/ArgParserTests.fs
index 40deba9..2e02bfd 100644
--- a/src/UniqueFileGenerator.Tests/ArgParserTests.fs
+++ b/src/UniqueFileGenerator.Tests/ArgParserTests.fs
@@ -18,13 +18,19 @@ let validOptionValues =
           Delay, "5_000" ]
 
 let defaultOptions =
-    { Prefix = Prefix.Create None |> _.Value
+    { Prefix = Prefix.Create None
+                |> function
+                | Ok x ->  x.Value
+                | Error e -> failwith $"Unexpected parse error: {e}"
       NameBaseLength =
           NameBaseLength.TryCreate None
           |> function
               | Ok x -> x.Value
               | Error e -> failwith $"Unexpected parse error: {e}"
-      Extension = Extension.Create None |> _.Value
+      Extension = Extension.Create None
+                  |> function
+                  | Ok x ->  x.Value
+                  | Error e -> failwith $"Unexpected parse error: {e}"
       OutputDirectory = OutputDirectory.Create None |> _.Value
       Size = Size.TryCreate None
              |> function
@@ -379,4 +385,3 @@ module SupportedSeparators =
         let result = stripSeparatorsAndTrim text
 
         Assert.Equal("helloworld\ttestexample\nmore", result)
-