diff --git a/async/if-else/docs/types/index.d.ts b/async/if-else/docs/types/index.d.ts
index 924c3f88..7dd425b4 100644
--- a/async/if-else/docs/types/index.d.ts
+++ b/async/if-else/docs/types/index.d.ts
@@ -44,7 +44,7 @@ type PredicateUnary = ( error: Error ) => void;
 * @param error - error object
 * @param bool - condition used to determine whether to invoke callback with `x` or `y`
 */
-type PredicateBinary = ( error: Error, bool: boolean ) => void;
+type PredicateBinary = ( error: Error | null, bool: boolean ) => void;
 
 /**
 * Predicate callback function.
diff --git a/async/if-then/docs/types/index.d.ts b/async/if-then/docs/types/index.d.ts
index a02fa739..48ecb13e 100644
--- a/async/if-then/docs/types/index.d.ts
+++ b/async/if-then/docs/types/index.d.ts
@@ -44,7 +44,7 @@ type PredicateUnary = ( error: Error ) => void;
 * @param error - error object
 * @param bool - condition used to determine whether to invoke `x` or `y`
 */
-type PredicateBinary = ( error: Error, bool: boolean ) => void;
+type PredicateBinary = ( error: Error | null, bool: boolean ) => void;
 
 /**
 * Predicate callback function.