diff --git a/Project.toml b/Project.toml
index 71676b81e..5dc4499a2 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "DataDrivenDiffEq"
 uuid = "2445eb08-9709-466a-b3fc-47e12bd697a2"
 authors = ["Julius Martensen <julius.martensen@gmail.com>"]
-version = "0.5.4"
+version = "0.5.5"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
diff --git a/src/DataDrivenDiffEq.jl b/src/DataDrivenDiffEq.jl
index 19816c832..f9f0da6ea 100644
--- a/src/DataDrivenDiffEq.jl
+++ b/src/DataDrivenDiffEq.jl
@@ -10,7 +10,7 @@ using FiniteDifferences, DataInterpolations
 using Compat
 using DocStringExtensions
 
-abstract type AbstractKoopmanOperator end;
+abstract type AbstractKoopmanOperator <: Function end;
 
 include("./optimizers/Optimize.jl")
 using .Optimize
@@ -85,6 +85,6 @@ export burst_sampling, subsample
 
 include("./basis_generators.jl")
 export chebyshev_basis, monomial_basis, polynomial_basis
-export sin_basis, cos_basis, fourier_basis 
+export sin_basis, cos_basis, fourier_basis
 
 end # module