diff --git a/gtsam/linear/PCGSolver.h b/gtsam/linear/PCGSolver.h
index 8d48da149..0201000ad 100644
--- a/gtsam/linear/PCGSolver.h
+++ b/gtsam/linear/PCGSolver.h
@@ -22,7 +22,7 @@ namespace gtsam {
 class GaussianFactorGraph;
 class KeyInfo;
 class Preconditioner;
-class PreconditionerParameters;
+struct PreconditionerParameters;
 
 /*****************************************************************************/
 struct GTSAM_EXPORT PCGSolverParameters: public ConjugateGradientParameters {