diff --git a/api.bs b/api.bs
index 6635ca0..d3b170b 100644
--- a/api.bs
+++ b/api.bs
@@ -1538,7 +1538,7 @@ To <dfn>validate {{AttributionConversionOptions}}</dfn> |options|:
     :: |credit|
 1.  Let |lookback| be |options|.{{AttributionConversionOptions/lookbackDays}} [=days=]
     if it [=map/exists=], the [=implementation-defined=] maximum otherwise.
-1.  Let |lookback| be the [=implementation-defined=] maximum
+1.  Set |lookback| to the [=implementation-defined=] maximum
     if it is larger than that maximum.
 1.  If |lookback| is 0 [=days=], throw a {{RangeError}}.
 1.  If the [=list/size=] of
diff --git a/impl/src/backend.ts b/impl/src/backend.ts
index bbe8c4e..fd5011f 100644
--- a/impl/src/backend.ts
+++ b/impl/src/backend.ts
@@ -276,6 +276,7 @@ export class Backend {
     if (lookbackDays <= 0 || !Number.isInteger(lookbackDays)) {
       throw new RangeError("lookbackDays must be a positive integer");
     }
+    lookbackDays = Math.min(lookbackDays, this.#delegate.maxLookbackDays);
 
     const matchValueSet = new Set<number>();
     for (const value of matchValues) {