diff --git a/panda/src/grutil/geoMipTerrain.I b/panda/src/grutil/geoMipTerrain.I
index 194f477e46..4f8d567369 100644
--- a/panda/src/grutil/geoMipTerrain.I
+++ b/panda/src/grutil/geoMipTerrain.I
@@ -361,9 +361,9 @@ set_heightfield(const Filename &filename, PNMFileType *ftype) {
   if (image.read(filename, ftype)) {
     _is_dirty = true;
     _heightfield = PNMImage(
-              max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_x_size())))
+              max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_x_size() - 1)))
                                                              / log(2.0))) + 1),
-              max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_y_size())))
+              max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_y_size() - 1)))
                                                              / log(2.0))) + 1));
     // Make sure not to apply gaussian when it's already the right size
     if (_heightfield.get_x_size() == image.get_x_size() &&
@@ -381,9 +381,9 @@ set_heightfield(const Filename &filename, PNMFileType *ftype) {
 INLINE bool GeoMipTerrain::
 set_heightfield(const PNMImage &image) {
   _heightfield = PNMImage(
-            max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_x_size())))
+            max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_x_size() - 1)))
                                                            / log(2.0))) + 1),
-            max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_y_size())))
+            max(3, (int) pow(2.0, ceil(log(float(max(2, image.get_y_size() - 1)))
                                                            / log(2.0))) + 1));
   // Make sure not to apply gaussian when it's already the right size
   if (_heightfield.get_x_size() == image.get_x_size() &&
@@ -400,9 +400,9 @@ set_heightfield(const PNMImage &image) {
 INLINE bool GeoMipTerrain::
 set_heightfield(const Texture *tex) {
   _heightfield = PNMImage(
-            max(3, (int) pow(2.0, ceil(log(float(max(2, tex->get_x_size())))
+            max(3, (int) pow(2.0, ceil(log(float(max(2, tex->get_x_size() - 1)))
                                                            / log(2.0))) + 1),
-            max(3, (int) pow(2.0, ceil(log(float(max(2, tex->get_y_size())))
+            max(3, (int) pow(2.0, ceil(log(float(max(2, tex->get_y_size() - 1)))
                                                            / log(2.0))) + 1));
   PNMImage image;
   tex->store(image);