diff --git a/crates/vm/src/builtins/str.rs b/crates/vm/src/builtins/str.rs
index b31dc6ccc9d..74564278925 100644
--- a/crates/vm/src/builtins/str.rs
+++ b/crates/vm/src/builtins/str.rs
@@ -1045,11 +1045,11 @@ impl PyStr {
         let mut swapped_str = Wtf8Buf::with_capacity(self.data.len());
         for c_orig in self.as_wtf8().code_points() {
             let c = c_orig.to_char_lossy();
-            // to_uppercase returns an iterator, to_ascii_uppercase returns the char
+            // to_uppercase returns an iterator because case changes may be multiple bytes
             if c.is_lowercase() {
-                swapped_str.push_char(c.to_ascii_uppercase());
+                swapped_str.extend(c.to_uppercase());
             } else if c.is_uppercase() {
-                swapped_str.push_char(c.to_ascii_lowercase());
+                swapped_str.extend(c.to_lowercase());
             } else {
                 swapped_str.push(c_orig);
             }
diff --git a/extra_tests/snippets/builtin_str.py b/extra_tests/snippets/builtin_str.py
index 12f97ac619d..b852e678ace 100644
--- a/extra_tests/snippets/builtin_str.py
+++ b/extra_tests/snippets/builtin_str.py
@@ -235,6 +235,8 @@
 assert not "😂".isidentifier()
 assert not "123".isidentifier()
 
+assert "Σίσυφος".swapcase() == "σΊΣΥΦΟΣ"
+
 # String Formatting
 assert "{} {}".format(1, 2) == "1 2"
 assert "{0} {1}".format(2, 3) == "2 3"