diff --git a/src/args/fraction.rs b/src/args/fraction.rs
index 8a1639fc..e7bef60a 100644
--- a/src/args/fraction.rs
+++ b/src/args/fraction.rs
@@ -18,12 +18,18 @@ pub enum ConversionError {
     /// Provided value is less than 0.
     #[display("less than 0")]
     LowerBound,
+    /// Provided value is `NaN`.
+    #[display("not a number")]
+    NotANumber,
 }
 
 impl Fraction {
     /// Create a [`Fraction`].
     pub fn new(value: f32) -> Result<Self, ConversionError> {
         use ConversionError::*;
+        if value.is_nan() {
+            return Err(NotANumber);
+        }
         if value >= 1.0 {
             return Err(UpperBound);
         }
diff --git a/tests/args_fraction.rs b/tests/args_fraction.rs
index d436b097..6413218f 100644
--- a/tests/args_fraction.rs
+++ b/tests/args_fraction.rs
@@ -48,6 +48,18 @@ fn equal_to_one() {
     );
 }
 
+#[test]
+fn not_a_number() {
+    let actual_error = "NaN".parse::<Fraction>().expect_err("cause nan error");
+    let actual_message = actual_error.to_string();
+    let expected_error = Conversion(NotANumber);
+    let expected_message = "not a number".to_string();
+    assert_eq!(
+        (actual_error, actual_message),
+        (expected_error, expected_message),
+    );
+}
+
 #[test]
 fn invalid_float_literal() {
     let actual = "a"