diff --git a/index.d.ts b/index.d.ts
index 5073226..0b72f17 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -167,6 +167,8 @@ export function RBTreeWrapper(...args: any[]): any;
 
 export function UnifyError(...args: any[]): any;
 
+export function NotAFunctionError(...args: any[]): any;
+
 export function addDouble(x: any): any;
 
 export function addInt(x: any): any;
diff --git a/lib/generics/generics.js b/lib/generics/generics.js
index df92250..63ce9e4 100644
--- a/lib/generics/generics.js
+++ b/lib/generics/generics.js
@@ -4,6 +4,7 @@ import { zip } from "../util/util.js";
 import { pretty, prettyT } from '../util/pretty.js';
 import { isTypeVar, TYPE_VARS } from "../primitives/typevars.js";
 import { inspectType } from "../meta/type_constructor.js";
+import { symbolFunction } from "../structures/type_constructors.js";
 
 // helper for creating generic types
 //   for instance, the type:
@@ -78,6 +79,7 @@ export const mergeTwoWay = (m1, m2) => {
 };
 
 export class UnifyError extends Error {}
+export class NotAFunctionError extends Error {}
 
 // Thanks to Hans for pointing out that this algorithm exactly like "Unification" in Prolog (hence the function name):
 //  https://www.dai.ed.ac.uk/groups/ssp/bookpages/quickprolog/node12.html
@@ -184,8 +186,10 @@ export const substitute = (type, substitutions, stack=[]) => {
   };
 };
 
-
 export const assignFn = (funType, paramType) => {
+  if (getSymbol(funType) !== symbolFunction) {
+    throw NotAFunctionError(`${prettyT(funType)} is not a function type!`);
+  }
   [funType, paramType] = recomputeTypeVars([funType, paramType]);
   // console.log(prettyT(funType), prettyT(paramType));
   const [inType, outType] = funType.params.map(p => p(funType));