diff --git a/src/parse.rs b/src/parse.rs
index 57bbe43..4e2106c 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -296,6 +296,9 @@ fn parse_unit(input: String) -> Result<NonMetric, ParseError> {
 		"us gal" => Ok(NonMetric::USGallon),
 
 		// Ambiguous units
+		"ton" => Err(ParseError::AmbiguousUnit(input, "short", "long")),
+		"tons" => Err(ParseError::AmbiguousUnit(input, "short", "long")),
+
 		"fluid ounce" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
 		"fluid ounces" => Err(ParseError::AmbiguousUnit(input, "imperial", "US")),
 		"fl oz" => Err(ParseError::AmbiguousUnit(input, "imp", "US")),
@@ -723,6 +726,11 @@ mod test {
 
 	#[test]
 	fn ambiguous_units() {
+		test_ambiguous_units(NonMetric::ShortTon, NonMetric::LongTon, &[
+			"ton",
+			"tons",
+		]);
+
 		test_ambiguous_units(NonMetric::ImperialFluidOunce, NonMetric::USFluidOunce, &[
 			"fluid ounce",
 			"fluid ounces",