diff --git a/build.rs b/build.rs
index 414eba9..a92fc24 100644
--- a/build.rs
+++ b/build.rs
@@ -15,6 +15,9 @@ fn main() {
     let ac = autocfg::new();
     ac.emit_rustc_version(1, 70);
 
+    // slice::fill
+    ac.emit_rustc_version(1, 50);
+
     // Option::zip
     ac.emit_rustc_version(1, 46);
 
diff --git a/src/impl_fmt.rs b/src/impl_fmt.rs
index 35ff216..8349193 100644
--- a/src/impl_fmt.rs
+++ b/src/impl_fmt.rs
@@ -367,7 +367,7 @@ fn format_ascii_digits_no_integer(
                 digits_ascii_be.copy_within(..sig_digit_count, sig_digit_idx);
 
                 // clear copied values
-                digits_ascii_be[..sig_digit_count.min(sig_digit_idx)].fill(b'0');
+                fill_slice(&mut digits_ascii_be[..sig_digit_count.min(sig_digit_idx)], b'0');
             } else {
                 debug_assert_eq!(sig_digit_count, 1);
             }
@@ -378,6 +378,18 @@ fn format_ascii_digits_no_integer(
     }
 }
 
+#[cfg(rust_1_50)]
+fn fill_slice<T: Clone>(v: &mut [T], value: T) {
+    v.fill(value);
+}
+
+#[cfg(not(rust_1_50))]
+fn fill_slice<T: Clone>(v: &mut [T], value: T) {
+    for i in v.iter_mut() {
+        *i = value.clone();
+    }
+}
+
 
 /// Format integer as {int}e+{exp}
 ///