diff --git a/python/_project.pyx b/python/_project.pyx
index 7e5eb89..3ccf7ad 100644
--- a/python/_project.pyx
+++ b/python/_project.pyx
@@ -22,7 +22,7 @@ cdef extern from "project_tool.hpp" namespace "":
 @cython.boundscheck(False)
 @cython.cdivision(True)
 @cython.wraparound(False)
-cdef int interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y, 
+cdef void interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y, 
                                         DTYPE_t z, 
                                         DTYPE_t[:,:,:] d, DTYPE_t Lbox, DTYPE_t *retval) nogil:
 
@@ -81,7 +81,7 @@ cdef int interp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y,
 @cython.boundscheck(False)
 @cython.cdivision(True)
 @cython.wraparound(False)
-cdef int ngp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y, 
+cdef void ngp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y, 
                                  DTYPE_t z, 
                                  DTYPE_t[:,:,:] d, DTYPE_t Lbox, DTYPE_t *retval) nogil:
 
@@ -111,7 +111,7 @@ cdef int ngp3d_INTERNAL_periodic(DTYPE_t x, DTYPE_t y,
 @cython.boundscheck(False)
 @cython.cdivision(True)
 @cython.wraparound(False)
-cdef int ngp3d_INTERNAL(DTYPE_t x, DTYPE_t y, 
+cdef void ngp3d_INTERNAL(DTYPE_t x, DTYPE_t y, 
                          DTYPE_t z, 
                          DTYPE_t[:,:,:] d, DTYPE_t Lbox, DTYPE_t *retval, DTYPE_t inval) nogil:
 
@@ -132,6 +132,7 @@ cdef int ngp3d_INTERNAL(DTYPE_t x, DTYPE_t y,
 
     if ((ix < 0)  or (ix+1) >= Ngrid or (iy < 0)  or (iy+1) >= Ngrid or (iz < 0)  or (iz+1) >= Ngrid):
       retval[0] = inval
+      return
 
     retval[0] = d[ix ,iy ,iz ] 
 
@@ -139,7 +140,7 @@ cdef int ngp3d_INTERNAL(DTYPE_t x, DTYPE_t y,
 @cython.boundscheck(False)
 @cython.cdivision(True)
 @cython.wraparound(False)
-cdef int interp3d_INTERNAL(DTYPE_t x, DTYPE_t y, 
+cdef void interp3d_INTERNAL(DTYPE_t x, DTYPE_t y, 
                                DTYPE_t z, 
                                DTYPE_t[:,:,:] d, DTYPE_t Lbox, DTYPE_t *retval, DTYPE_t inval) nogil:
     
@@ -163,6 +164,7 @@ cdef int interp3d_INTERNAL(DTYPE_t x, DTYPE_t y,
 
     if ((ix < 0)  or (ix+1) >= Ngrid or (iy < 0)  or (iy+1) >= Ngrid or (iz < 0)  or (iz+1) >= Ngrid):
       retval[0] = inval
+      return
     #   assert ((ix >= 0) and ((ix+1) < Ngrid))
 #    assert ((iy >= 0) and ((iy+1) < Ngrid))
 #    assert ((iz >= 0) and ((iz+1) < Ngrid))