diff --git a/.github/workflows/clippy.yml b/.github/workflows/clippy.yml
index 1ddcf5f..e46eb6d 100644
--- a/.github/workflows/clippy.yml
+++ b/.github/workflows/clippy.yml
@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1
         with:
-            toolchain: nightly
+            toolchain: nightly-2021-05-16
             components: clippy
             override: true
       - uses: actions-rs/clippy-check@v1
diff --git a/.github/workflows/rust.yml b/.github/workflows/rust.yml
index 0b30106..cbc1370 100644
--- a/.github/workflows/rust.yml
+++ b/.github/workflows/rust.yml
@@ -18,7 +18,7 @@ jobs:
       - name: Set nightly toolchain
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: nightly-2021-05-16
           override: true
       - name: Environment
         run: |
diff --git a/src/helper.rs b/src/helper.rs
index 2ab56f7..0de5722 100644
--- a/src/helper.rs
+++ b/src/helper.rs
@@ -113,6 +113,7 @@ mod html {
 
     pub fn superscript(n: u8) -> String {
         match n {
+            x if x >= 10 => format!("{}{}", superscript(n / 10), superscript(n % 10)),
             0 => "⁰".to_string(),
             1 => "¹".to_string(),
             2 => "²".to_string(),
@@ -123,18 +124,13 @@ mod html {
             7 => "⁷".to_string(),
             8 => "⁸".to_string(),
             9 => "⁹".to_string(),
-            x if x > 10 => (superscript(n / 10).parse().unwrap_or(0)
-                + superscript(n % 10).parse().unwrap_or(0))
-            .to_string(),
             _ => n.to_string(),
         }
     }
 
     pub fn subscript(n: u8) -> String {
         match n {
-            x if x >= 10 => (subscript(n / 10).parse().unwrap_or(0)
-                + subscript(n % 10).parse().unwrap_or(0))
-            .to_string(),
+            x if x >= 10 => format!("{}{}", subscript(n / 10), subscript(n % 10)),
             0 => "₀".to_string(),
             1 => "₁".to_string(),
             2 => "₂".to_string(),