diff --git a/README.md b/README.md
index 19458c3..78c46d9 100644
--- a/README.md
+++ b/README.md
@@ -350,7 +350,6 @@ This crate offers a set of builtin functions.
 | `is_finite`          | 1               | Numeric                | Returns true if the argument is a finite floating-point number, false otherwise  |
 | `is_infinite`        | 1               | Numeric                | Returns true if the argument is an infinite floating-point number, false otherwise  |
 | `is_normal`          | 1               | Numeric                | Returns true if the argument is a floating-point number that is neither zero, infinite, subnormal, or NaN, false otherwise  |
-| `is_subnormal`       | 1               | Numeric                | Returns true if the argument is a [subnormal](https://en.wikipedia.org/wiki/Subnormal_number) number, false otherwise  |
 | `math::ln`           | 1               | Numeric                | Returns the natural logarithm of the number |
 | `math::log`          | 2               | Numeric, Numeric       | Returns the logarithm of the number with respect to an arbitrary base |
 | `math::log2`         | 1               | Numeric                | Returns the base 2 logarithm of the number |
diff --git a/src/function/builtin.rs b/src/function/builtin.rs
index 3b6d286..95aca35 100644
--- a/src/function/builtin.rs
+++ b/src/function/builtin.rs
@@ -90,7 +90,6 @@ pub fn builtin_function(identifier: &str) -> Option<Function> {
         "is_finite" => float_is(f64::is_finite),
         "is_infinite" => float_is(f64::is_infinite),
         "is_normal" => float_is(f64::is_normal),
-        "is_subnormal" => float_is(f64::is_subnormal),
         // Other
         "min" => Some(Function::new(|argument| {
             let arguments = argument.as_tuple()?;
diff --git a/src/lib.rs b/src/lib.rs
index 237483e..0391408 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -331,8 +331,7 @@
 //! | `is_nan`             | 1               | Numeric                | Returns true if the argument is the floating-point value NaN, false otherwise  |
 //! | `is_finite`          | 1               | Numeric                | Returns true if the argument is a finite floating-point number, false otherwise  |
 //! | `is_infinite`        | 1               | Numeric                | Returns true if the argument is an infinite floating-point number, false otherwise  |
-//! | `is_normal`          | 1               | Numeric                | Returns true if the argument is a floating-point number that is neither zero, infinite, subnormal, or NaN, false otherwise  |
-//! | `is_subnormal`       | 1               | Numeric                | Returns true if the argument is a [subnormal](https://en.wikipedia.org/wiki/Subnormal_number) number, false otherwise  |
+//! | `is_normal`          | 1               | Numeric                | Returns true if the argument is a floating-point number that is neither zero, infinite, [subnormal](https://en.wikipedia.org/wiki/Subnormal_number), or NaN, false otherwise  |
 //! | `math::ln`           | 1               | Numeric                | Returns the natural logarithm of the number |
 //! | `math::log`          | 2               | Numeric, Numeric       | Returns the logarithm of the number with respect to an arbitrary base |
 //! | `math::log2`         | 1               | Numeric                | Returns the base 2 logarithm of the number |
diff --git a/tests/integration.rs b/tests/integration.rs
index 77fc9fe..63850bb 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -373,7 +373,6 @@ fn test_builtin_functions() {
     assert_eq!(eval("is_normal(1.0/0.0)"), Ok(Value::Boolean(false)));
     assert_eq!(eval("is_normal(0)"), Ok(Value::Boolean(false)));
     assert_eq!(eval("is_subnormal(0)"), Ok(Value::Boolean(false)));
-    assert_eq!(eval("is_subnormal(1.0e-308)"), Ok(Value::Boolean(true)));
     // Other
     assert_eq!(eval("min(4.0, 3)"), Ok(Value::Int(3)));
     assert_eq!(eval("max(4.0, 3)"), Ok(Value::Float(4.0)));