diff --git a/.changeset/eleven-kids-decide.md b/.changeset/eleven-kids-decide.md
new file mode 100644
index 00000000000..d93cb9247da
--- /dev/null
+++ b/.changeset/eleven-kids-decide.md
@@ -0,0 +1,5 @@
+---
+"@smithy/types": patch
+---
+
+fix: broken ChecksumConfiguration interface in TS < 4.4 and conditional generic types in TS<4.1
diff --git a/packages/types/package.json b/packages/types/package.json
index f3916f188b5..c48286b239e 100644
--- a/packages/types/package.json
+++ b/packages/types/package.json
@@ -30,7 +30,7 @@
     "node": ">=14.0.0"
   },
   "typesVersions": {
-    "<4.0": {
+    "<=4.0": {
       "dist-types/*": [
         "dist-types/ts3.4/*"
       ]
diff --git a/packages/types/src/extensions/checksum.ts b/packages/types/src/extensions/checksum.ts
index 42e16c2eeb8..1eb488e7ff1 100644
--- a/packages/types/src/extensions/checksum.ts
+++ b/packages/types/src/extensions/checksum.ts
@@ -21,16 +21,21 @@ export interface ChecksumAlgorithm {
 }
 
 /**
- * @internal
+ * @deprecated unused.
  */
-export interface ChecksumConfiguration {
-  addChecksumAlgorithm(algo: ChecksumAlgorithm): void;
-  checksumAlgorithms(): ChecksumAlgorithm[];
-
+type ChecksumConfigurationLegacy = {
   /**
    * @deprecated unused.
    */
-  [other: string | number]: any;
+  [other in string | number]: any;
+};
+
+/**
+ * @internal
+ */
+export interface ChecksumConfiguration extends ChecksumConfigurationLegacy {
+  addChecksumAlgorithm(algo: ChecksumAlgorithm): void;
+  checksumAlgorithms(): ChecksumAlgorithm[];
 }
 
 /**