diff --git a/CHANGELOG.md b/CHANGELOG.md
index de08153e81..f42d038c0b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Changes:
 
 - Support for Metadata v12 with fixed indices
 - Cleanups for WebSocket class detection and creation
+- Ensure that ignored map params yield an error on `iterKey`
 
 
 ## 1.33.1 Sep 14, 2020
diff --git a/packages/metadata/src/Decorated/storage/fromMetadata/createFunction.ts b/packages/metadata/src/Decorated/storage/fromMetadata/createFunction.ts
index 23efef4870..c4865a2707 100644
--- a/packages/metadata/src/Decorated/storage/fromMetadata/createFunction.ts
+++ b/packages/metadata/src/Decorated/storage/fromMetadata/createFunction.ts
@@ -181,11 +181,13 @@ function extendLinkedMap (registry: Registry, itemFn: CreateItemFn, storageFn: S
 function extendPrefixedMap (registry: Registry, itemFn: CreateItemFn, storageFn: StorageEntry): StorageEntry {
   const { meta: { type } } = itemFn;
 
-  storageFn.iterKey = extendHeadMeta(registry, itemFn, storageFn, (arg?: any): Raw =>
-    type.isDoubleMap && !isUndefined(arg) && !isNull(arg)
+  storageFn.iterKey = extendHeadMeta(registry, itemFn, storageFn, (arg?: any): Raw => {
+    assert(type.isDoubleMap || isUndefined(arg), 'Filtering arguments for keys/entries are only valid on double maps');
+
+    return type.isDoubleMap && !isUndefined(arg) && !isNull(arg)
       ? new Raw(registry, u8aConcat(createPrefixedKey(itemFn), getHasher(type.asDoubleMap.hasher)(registry.createType(type.asDoubleMap.key1.toString() as 'Raw', arg).toU8a())))
-      : new Raw(registry, createPrefixedKey(itemFn))
-  );
+      : new Raw(registry, createPrefixedKey(itemFn));
+  });
 
   return storageFn;
 }