diff --git a/Cargo.lock b/Cargo.lock
index f3d30e1..74ffa4f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -505,9 +505,9 @@ dependencies = [
 
 [[package]]
 name = "redox_syscall"
-version = "0.2.6"
+version = "0.2.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8270314b5ccceb518e7e578952f0b72b88222d02e8f77f5ecf7abbb673539041"
+checksum = "85dd92e586f7355c633911e11f77f3d12f04b1b1bd76a198bd34ae3af8341ef2"
 dependencies = [
  "bitflags",
 ]
diff --git a/src/parameters.rs b/src/parameters.rs
index 00d047a..d68bdcf 100644
--- a/src/parameters.rs
+++ b/src/parameters.rs
@@ -48,7 +48,7 @@ pub struct Parameters {
 	/// Only examine files with these extensions.
 	/// Multiple extensions can be specified by either using the flag multiple times (`-e jpg -e png -e gif`), or by
 	/// separating them with commas (`-e jpg,png,gif`).
-	#[clap(short, long, use_delimiter = true, require_delimiter = true, value_name = "ext")]
+	#[clap(short, long, use_delimiter = true, require_delimiter = true, value_name = "ext", validator = lowercase_exts)]
 	pub exts: Option<Vec<StringType>>,
 
 	/// Use these preset lists of extensions as the search filter (comma-separated list).
@@ -59,7 +59,7 @@ pub struct Parameters {
 
 	/// Don't scan files with these extensions.
 	/// This option takes precedence over extensions specified with `-e` or `-E`.
-	#[clap(short = 'x', long, use_delimiter = true, require_delimiter = true, value_name = "ext")]
+	#[clap(short = 'x', long, use_delimiter = true, require_delimiter = true, value_name = "ext", validator = lowercase_exts)]
 	pub exclude: Option<Vec<StringType>>,
 
 	/// Exclude files using a preset list of extensions.
@@ -103,6 +103,14 @@ pub struct Parameters {
 	pub dirs: PathBuf,
 }
 
+fn lowercase_exts(exts: &str) -> Result<(), String> {
+	// TODO: i would much rather accept uppercase exts and convert them to lowercase than just rejecting lowercase exts...
+	if exts.to_lowercase() != exts {
+		return Err(String::from("Supplied extensions must be lowercase"))
+	}
+	Ok(())
+}
+
 /// Further options relating to scanning.
 #[derive(PartialEq, Debug)]
 pub struct ScanOpts {