Patch kindly borrowed from Debian.
Gentoo-bug: https://bugs.gentoo.org/574746
KDE-bug: https://bugs.kde.org/show_bug.cgi?id=355324

--- a/src/plugins/fits/non_linear.h
+++ b/src/plugins/fits/non_linear.h
@@ -177,7 +177,9 @@ bool kstfit_nonlinear(
             }
             iIterations++;
           } while( iStatus == GSL_CONTINUE && iIterations < MAX_NUM_ITERATIONS );
-          gsl_multifit_covar( pSolver->J, 0.0, pMatrixCovariance );
+          gsl_matrix *J = gsl_matrix_alloc (pSolver->fdf->n, pSolver->fdf->p);
+          gsl_multifit_fdfsolver_jac (pSolver, J);
+          gsl_multifit_covar (J, 0.0, pMatrixCovariance);
 
           //
           // determine the fitted values...
--- a/src/plugins/fits/non_linear_weighted.h
+++ b/src/plugins/fits/non_linear_weighted.h
@@ -193,7 +193,9 @@ bool kstfit_nonlinear_weighted(
           }
           while( iStatus == GSL_CONTINUE && iIterations < MAX_NUM_ITERATIONS );
 
-          gsl_multifit_covar( pSolver->J, 0.0, pMatrixCovariance );
+          gsl_matrix *J = gsl_matrix_alloc (pSolver->fdf->n, pSolver->fdf->p);
+          gsl_multifit_fdfsolver_jac (pSolver, J);
+          gsl_multifit_covar (J, 0.0, pMatrixCovariance);
 
           //
           // determine the fitted values...