diff --git a/unsupported/Eigen/src/NonLinear/HybridNonLinearSolver.h b/unsupported/Eigen/src/NonLinear/HybridNonLinearSolver.h
index 6022ce363..bdca46cd4 100644
--- a/unsupported/Eigen/src/NonLinear/HybridNonLinearSolver.h
+++ b/unsupported/Eigen/src/NonLinear/HybridNonLinearSolver.h
@@ -113,6 +113,7 @@ HybridNonLinearSolver<FunctorType,Scalar>::solve(
     fvec.resize(n);
     if (mode != 2)
         diag.resize(n);
+    assert( (mode!=2 || diag.size()==n) || "When using mode==2, the caller must provide a valid 'diag'");
 
     /* Local variables */
     int i, j, l, iwa[1];
@@ -433,6 +434,7 @@ HybridNonLinearSolver<FunctorType,Scalar>::solveNumericalDiff(
     fvec.resize(n);
     if (mode != 2)
         diag.resize(n);
+    assert( (mode!=2 || diag.size()==n) || "When using mode==2, the caller must provide a valid 'diag'");
 
     /* Local variables */
     int i, j, l, iwa[1];
diff --git a/unsupported/Eigen/src/NonLinear/LevenbergMarquardt.h b/unsupported/Eigen/src/NonLinear/LevenbergMarquardt.h
index 35b6e3ee2..96a90810e 100644
--- a/unsupported/Eigen/src/NonLinear/LevenbergMarquardt.h
+++ b/unsupported/Eigen/src/NonLinear/LevenbergMarquardt.h
@@ -132,6 +132,7 @@ LevenbergMarquardt<FunctorType,Scalar>::minimize(
     fjac.resize(m, n);
     if (mode != 2)
         diag.resize(n);
+    assert( (mode!=2 || diag.size()==n) || "When using mode==2, the caller must provide a valid 'diag'");
     qtf.resize(n);
 
     /* Local variables */
@@ -410,6 +411,7 @@ LevenbergMarquardt<FunctorType,Scalar>::minimizeNumericalDiff(
     fjac.resize(m, n);
     if (mode != 2 )
         diag.resize(n);
+    assert( (mode!=2 || diag.size()==n) || "When using mode==2, the caller must provide a valid 'diag'");
     qtf.resize(n);
 
     /* Local variables */
@@ -688,6 +690,7 @@ LevenbergMarquardt<FunctorType,Scalar>::minimizeOptimumStorage(
     fjac.resize(m, n);
     if (mode != 2)
         diag.resize(n);
+    assert( (mode!=2 || diag.size()==n) || "When using mode==2, the caller must provide a valid 'diag'");
     qtf.resize(n);
 
     /* Local variables */