diff --git a/src/SparseMatrixIdentification.jl b/src/SparseMatrixIdentification.jl
index 27aa354..d0a6aa2 100644
--- a/src/SparseMatrixIdentification.jl
+++ b/src/SparseMatrixIdentification.jl
@@ -101,7 +101,7 @@ end
 export getstructure
 
 # get the percentage banded for a bandwidth of 1 and percentage sparsity
-function getstructure(A::Matrix)
+function getstructure(A::AbstractMatrix)::Any
     percentage_banded = compute_bandedness(A, 1)
     percentage_sparsity = compute_sparsity(SparseMatrixCSC(A))