diff --git a/AutoDiffHelpers.hpp b/AutoDiffHelpers.hpp
index f48b6e165..740e8b8ba 100644
--- a/AutoDiffHelpers.hpp
+++ b/AutoDiffHelpers.hpp
@@ -286,6 +286,8 @@ superset(const Eigen::Array<Scalar, Eigen::Dynamic, 1>& x,
 
 /// Construct square sparse matrix with the
 /// elements of d on the diagonal.
+/// Need to mark this as inline since it is defined in a header and not a template.
+inline
 AutoDiff::ForwardBlock<double>::M
 spdiag(const AutoDiff::ForwardBlock<double>::V& d)
 {