diff --git a/index.d.ts b/index.d.ts
index d0b4f65..a6fe6e2 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -165,8 +165,12 @@ export const unit: {
 
 export function RBTreeWrapper(...args: any[]): any;
 
-export class IncompatibleTypesError extends Error {};
-export class SubstitutionCycle extends Error {};
+export class IncompatibleTypesError extends Error {
+    constructor(typeA: any, typeB: any, nestedErr: Error);
+};
+export class SubstitutionCycle extends Error {
+    constructor(typevar: string, type: any);
+};
 
 export function addDouble(x: any): any;