diff --git a/src/types/string.rs b/src/types/string.rs
index 031598a90bf..dd29a5400bc 100644
--- a/src/types/string.rs
+++ b/src/types/string.rs
@@ -8,6 +8,7 @@ use crate::{
     Python, ToPyObject,
 };
 use std::borrow::Cow;
+use std::ffi::CStr;
 use std::os::raw::c_char;
 use std::str;
 
@@ -236,13 +237,13 @@ impl PyString {
             (0..len).map(move |i| {
                 let c = ffi::PyUnicode_ReadChar(self.as_ptr(), i);
                 char::from_u32(c).ok_or_else(|| {
-                    exceptions::PyUnicodeDecodeError::new_err((
-                        "utf-32",
-                        PyBytes::new(self.py(), &c.to_ne_bytes()).to_object(self.py()),
-                        i,
-                        i + 1,
-                        format!("invalid codepoint"),
-                    ))
+                    exceptions::PyUnicodeDecodeError::new(
+                        self.py(),
+                        CStr::from_bytes_with_nul(b"utf-8\0").unwrap(),
+                        &c.to_ne_bytes(),
+                        i as usize..(i + 1) as usize,
+                        CStr::from_bytes_with_nul(b"invalid codepoint\0").unwrap(),
+                    )
                 })
             })
         }