diff --git a/src/splpak.F90 b/src/splpak.F90
index 25264a6..0aa691d 100644
--- a/src/splpak.F90
+++ b/src/splpak.F90
@@ -308,7 +308,7 @@ subroutine bascmp(me,x,nderiv,xmin,nodes,icol,basm)
                 fact = me%dxin(idim)
             end if
             z = fact* (x(idim)-xb) + 2.0_wp
-            if (z<0.0_wp) then
+            if (z>0.0_wp) then
                 if (z<2.0_wp) then
                     bas1 = 1.5_wp*z**2
                     z = z - 1.0_wp
diff --git a/test/splpak_test_linear.f90 b/test/splpak_test_linear.f90
new file mode 100644
index 0000000..f522114
--- /dev/null
+++ b/test/splpak_test_linear.f90
@@ -0,0 +1,114 @@
+!*****************************************************************************************
+!>
+!  Units test for splpak.
+!  Test a linear function
+
+    program splpak_test_linesr
+
+    use splpak_module, wp => splpak_wp
+    use iso_fortran_env
+    use pyplot_module
+
+    implicit none
+
+    integer,parameter :: ndim = 1    !! 1d problem
+    integer,parameter :: nxdata = 20 !! number of points in x
+    integer,dimension(ndim),parameter :: nodes = [10]
+    integer,parameter :: ncol = product(nodes)
+    integer,parameter :: nwrk = ncol*(ncol+1) + 1   ! is doc wrong? do we need the + 1 (otherwise, it fails)...
+    integer,parameter :: ncf = ncol
+    integer,parameter :: nxdata_est = 100 !! number of points for estimate plot
+
+    real(wp),dimension(ndim,nxdata) :: xdata
+    real(wp),dimension(nxdata) :: ydata
+    real(wp),dimension(ndim)   :: xmin
+    real(wp),dimension(ndim)   :: xmax
+    real(wp),dimension(nwrk)   :: work
+    real(wp),dimension(ncf)    :: coef
+    real(wp),dimension(nxdata) :: wdata !! weights
+    real(wp),dimension(nxdata_est) :: xdata_est, ydata_est
+    real(wp),dimension(ndim)   :: x
+
+    integer :: ierror, istat
+    integer :: i !! counter
+    real(wp) :: xtrap
+    real(wp) :: tru, err, errmax, f, fleft, fright
+    type(pyplot) :: plt
+    character(len=10) :: nodes_str !! string version of `nodes`
+    type(splpak_type) :: solver, solver2
+    integer,dimension(2),parameter :: figsize = [20,10] !! figure size for plot
+
+    xtrap = 1.0_wp
+    xmin(1) = 0.0_wp
+    xmax(1) = 1.0_wp
+    do i=1,nxdata
+        wdata(i) = 1.0_wp   ! weight
+        xdata(1,i) = real(i-1,wp)/real(nxdata-1,wp)
+        ydata(i) = f1(xdata(1,i))
+    end do
+
+    ! write(*,'(a,*(f8.3,","))') 'xdata = ', xdata
+    ! write(*,'(a,*(f8.3,","))') 'ydata = ', ydata
+
+    ! initialize:
+    call solver%initialize(1,xdata,1,ydata,wdata,nxdata,xmin,xmax, &
+                           nodes,xtrap,coef,ncf,work,nwrk,ierror)
+
+    write(*,*) 'splcw ierror = ', ierror
+    if (ierror /= 0) error stop 'error calling splcw'
+
+    ! compute max error at interpolation points
+    errmax = 0.0_wp
+    do i=1,nxdata_est
+        xdata_est(i) = real(i-1,wp)/nxdata_est
+        x(1) = xdata_est(i)
+        f = solver%evaluate(ndim,x,coef,xmin,xmax,nodes,ierror)
+        ydata_est(i) = f
+        if (ierror /= 0) error stop 'error calling splfe'
+        tru    = f1(xdata_est(i))
+        err    = abs(tru-f)
+        errmax = max(err,errmax)
+    end do
+    write(*,*) 'splfe errmax [linear] = ', errmax
+    if (abs(errmax)>1.0e-1_wp) error stop 'errmax too large'
+
+    ! write(*,'(a,*(f8.3,","))') 'xdata_est = ', xdata_est
+    ! write(*,'(a,*(f8.3,","))') 'ydata_est = ', ydata_est
+
+    ! test the derivatives also:
+    fleft = solver%evaluate(ndim,[0.0_wp],[1],coef,xmin,xmax,nodes,ierror)
+    if (ierror /= 0) error stop 'error calling splde'
+    errmax = fleft - 2.0_wp
+    write(*,*) 'splde errmax left [linear] = ', errmax
+    if (abs(errmax)>1.0e-12_wp) error stop 'errmax too large'
+
+    fright = solver%evaluate(ndim,[1.0_wp],[1],coef,xmin,xmax,nodes,ierror)
+    if (ierror /= 0) error stop 'error calling splde'
+    errmax = fleft - 2.0_wp
+    write(*,*) 'splde errmax right [linear] = ', errmax
+    if (abs(errmax)>1.0e-12_wp) error stop 'errmax too large'
+
+    write(nodes_str,'(I10)') nodes(1); nodes_str = adjustl(nodes_str)
+
+    call plt%initialize(grid=.true.,xlabel='x',ylabel='y',&
+                        figsize=figsize,font_size=20,axes_labelsize=20,&
+                        xtick_labelsize=20, ytick_labelsize=20,&
+                        legend_fontsize=20,&
+                        title='splpak_test_linear',legend=.true.)
+    call plt%add_plot(xdata(1,:),ydata,&
+                        label='Original points',&
+                        linestyle='ko',markersize=5,linewidth=2,istat=istat)
+    call plt%add_plot(xdata_est,ydata_est,&
+                        label='Least squares bspline with '//trim(nodes_str)//' nodes',&
+                        linestyle='r-',markersize=2,linewidth=2,istat=istat)
+    call plt%savefig(pyfile='splpak_test.py', figfile='splpak_test_linear.png',istat=istat)
+
+    contains
+
+        real(wp) function f1(x) !! 1d test function
+        implicit none
+        real(wp) :: x
+        f1 = 2.0_wp * x
+        end function f1
+
+    end program splpak_test_linesr
\ No newline at end of file