diff --git a/gap/nofoma.gd b/gap/nofoma.gd
index bb6f6df..e978baa 100644
--- a/gap/nofoma.gd
+++ b/gap/nofoma.gd
@@ -375,7 +375,7 @@ DeclareGlobalFunction("JordanChevalleyDecMatF");
 #! @Description
 #!  Returns a list containing three elements. The first element is
 #!  a base change matrix <M>B</M> such that <M>B</M><A>A</A><M>B^{-1}</M> is a
-#!  primary form of <A>A</A>, i.e., a block diagonal matrix where the minimal polynomials
+#!  primary form of the matrix <A>A</A>, i.e., a block diagonal matrix where the minimal polynomials
 #!  of the the diagonal blocks are precisely the powers of irreducible factors
 #!  of the minimal polynomial of <A>A</A>, in descending order. The second element is a list containing
 #!  the collected irreducible factors of the minimal polynomial of <A>A</A>, in the same order. The
@@ -399,7 +399,7 @@ DeclareGlobalFunction("JordanChevalleyDecMatF");
 #! gap> MinimalPolynomial(PrimA{[2..5]}{[2..5]});
 #! x_1^4-x_1^3+Z(5)^3*x_1+Z(5)^3
 #! @EndExampleSession
-DeclareGlobalFunction("PrimaryDecomposition");
+DeclareAttribute("PrimaryDecomposition", IsMatrixOrMatrixObj);
 
 #! @Chapter Auxiliary functions
 #! @Section Vectors and matrices and their associated polynomials
diff --git a/gap/nofoma.gi b/gap/nofoma.gi
index 32b1e7e..0b0c037 100644
--- a/gap/nofoma.gi
+++ b/gap/nofoma.gi
@@ -759,7 +759,9 @@ end);
 #Standalone version
 #Returns matrix B such that B*A*B^-1 is in primary decomposition form
 #along with dimensions of primary subspaces
-InstallGlobalFunction(PrimaryDecomposition, function(A)
+InstallMethod(PrimaryDecomposition,
+    [ IsMatrixOrMatrixObj ],
+function(A)
     local rank,F,n,m,f,w,p,j,i,wspan,gens,facs,L_i,qi,k,v,
     COB,pot,gs,f2,dims,toAdd,dim,minpol,collected;
     rank := 0;