diff --git a/src/_ChangeBasis.jl b/src/_ChangeBasis.jl
index 26b05686b..94c2517dc 100644
--- a/src/_ChangeBasis.jl
+++ b/src/_ChangeBasis.jl
@@ -459,7 +459,7 @@ function _changebasis_I(P::BSplineSpace{p,T,<:AbstractKnotVector{T}}, P′::BSpl
     K′ = [k′[j+p′] - k′[j] for j in 1:n′+1]
     K = U[ifelse(k[i+p] ≠ k[i], U(1 / (k[i+p] - k[i])), zero(U)) for i in 1:n+1]
     Aᵖ⁻¹ = _changebasis_I(_lower_I(P), _lower_I(P′))  # (n-1) × (n′-1) matrix
-    n_nonzero = exactdim_I(P′)*(p+1)  # This is a upper bound of the number of non-zero elements of Aᵖ (rough estimation).
+    n_nonzero = exactdim_I(P′)*(p+2)  # This would be a upper bound of the number of non-zero elements of Aᵖ.
     I = Vector{Int32}(undef, n_nonzero)
     J = Vector{Int32}(undef, n_nonzero)
     V = Vector{U}(undef, n_nonzero)