diff --git a/.changeset/ten-lobsters-juggle.md b/.changeset/ten-lobsters-juggle.md
new file mode 100644
index 00000000000..23fb91e085b
--- /dev/null
+++ b/.changeset/ten-lobsters-juggle.md
@@ -0,0 +1,5 @@
+---
+"@smithy/util-endpoints": minor
+---
+
+return empty when given non-ASCII input in substring
diff --git a/packages/util-endpoints/src/lib/substring.spec.ts b/packages/util-endpoints/src/lib/substring.spec.ts
index a2e845da8a6..38974feb2c9 100644
--- a/packages/util-endpoints/src/lib/substring.spec.ts
+++ b/packages/util-endpoints/src/lib/substring.spec.ts
@@ -12,6 +12,12 @@ describe(substring.name, () => {
     it("when input.length < stop", () => {
       expect(substring("", 0, 1, false)).toBeNull();
     });
+
+    it("when input contains non-ASCII characters", () => {
+      expect(substring("abc\u0080", 0, 3, false)).toBeNull();
+      expect(substring("abcé", 0, 3, false)).toBeNull();
+      expect(substring("ab日c", 0, 3, false)).toBeNull();
+    });
   });
 
   it("returns substring", () => {
diff --git a/packages/util-endpoints/src/lib/substring.ts b/packages/util-endpoints/src/lib/substring.ts
index 62e46bb33c5..5c3a274b016 100644
--- a/packages/util-endpoints/src/lib/substring.ts
+++ b/packages/util-endpoints/src/lib/substring.ts
@@ -5,7 +5,7 @@
  * The length of the returned string will always be stop-start.
  */
 export const substring = (input: string, start: number, stop: number, reverse: boolean): string | null => {
-  if (start >= stop || input.length < stop) {
+  if (start >= stop || input.length < stop || /[^\u0000-\u007f]/.test(input)) {
     return null;
   }
   if (!reverse) {