diff --git a/lib/generics/generics.js b/lib/generics/generics.js
index 63ce9e4..a722a1f 100644
--- a/lib/generics/generics.js
+++ b/lib/generics/generics.js
@@ -188,7 +188,7 @@ export const substitute = (type, substitutions, stack=[]) => {
 
 export const assignFn = (funType, paramType) => {
   if (getSymbol(funType) !== symbolFunction) {
-    throw NotAFunctionError(`${prettyT(funType)} is not a function type!`);
+    throw new NotAFunctionError(`${prettyT(funType)} is not a function type!`);
   }
   [funType, paramType] = recomputeTypeVars([funType, paramType]);
   // console.log(prettyT(funType), prettyT(paramType));