diff --git a/lang/funcs/simple/simple.go b/lang/funcs/simple/simple.go
index 44d006df..03f7b39e 100644
--- a/lang/funcs/simple/simple.go
+++ b/lang/funcs/simple/simple.go
@@ -48,6 +48,9 @@ func Register(name string, fn *types.FuncValue) {
 	if fn.T == nil {
 		panic(fmt.Sprintf("simple func %s contains a nil type signature", name))
 	}
+	if fn.T.Kind != types.KindFunc {
+		panic(fmt.Sprintf("simple func %s must be of kind func", name))
+	}
 	if fn.T.HasVariant() {
 		panic(fmt.Sprintf("simple func %s contains a variant type signature", name))
 	}