diff --git a/Cargo.lock b/Cargo.lock
index e7e4c08..53ce67b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -30,12 +30,6 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d2fabcfbdc87f4758337ca535fb41a6d701b65693ce38287d856d1674551ec9b"
 
-[[package]]
-name = "heck"
-version = "0.4.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "95505c38b4572b2d910cecb0281560f54b440a19336cbbcb27bf6ce6adc6f5a8"
-
 [[package]]
 name = "itoa"
 version = "1.0.10"
@@ -76,7 +70,6 @@ checksum = "f98d2aa92eebf49b69786be48e4477826b256916e84a57ff2a4f21923b48eb4c"
 name = "sealed"
 version = "0.5.0"
 dependencies = [
- "heck",
  "proc-macro2",
  "quote",
  "syn",
diff --git a/Cargo.toml b/Cargo.toml
index fb2173d..f439e12 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -27,4 +27,3 @@ syn = { version = "2.0", features = ["extra-traits"] }
 syn = { version = "2.0", features = ["full"] }
 quote = "1.0"
 proc-macro2 = "1.0"
-heck = "0.4"
diff --git a/src/lib.rs b/src/lib.rs
index 8fc771b..93efff3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -119,7 +119,6 @@
 
 use std::fmt;
 
-use heck::ToSnakeCase as _;
 use proc_macro::TokenStream;
 use proc_macro2::{Span, TokenStream as TokenStream2};
 use quote::{format_ident, quote};
@@ -216,9 +215,29 @@ fn parse_sealed_impl(item_impl: &syn::ItemImpl) -> syn::Result<TokenStream2> {
     })
 }
 
+/// Convert a string into snake case.
+///
+/// Stolen't from <https://github.com/jmg-duarte/sealed-rs/pull/6#pullrequestreview-653837118>
+fn to_snake_case(s: &'_ str) -> String {
+    let mut ret = String::with_capacity(s.len());
+    let mut first = true;
+    s.bytes().for_each(|c| {
+        if c.is_ascii_uppercase() {
+            if !first {
+                ret.push('_');
+            }
+            ret.push(c.to_ascii_lowercase() as char);
+        } else {
+            ret.push(c as char);
+        }
+        first = false;
+    });
+    ret
+}
+
 /// Constructs [`syn::Ident`] of a sealing module name.
 fn seal_name<D: fmt::Display>(seal: D) -> syn::Ident {
-    format_ident!("__seal_{}", &seal.to_string().to_snake_case())
+    format_ident!("__seal_{}", to_snake_case(&seal.to_string()))
 }
 
 /// Arguments accepted by `#[sealed]` attribute when placed on a trait