diff --git a/NEWS.md b/NEWS.md
index fd7ec25b7..be59704ae 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -18,7 +18,8 @@ rearrangements of Notcurses.
     `ncplane_as_rgba()`, `ncplane_contents()`, and `ncvisual_from_plane()`,
     which all used -1. A length of zero passed to line-drawing functions is
     now an error. Several line-drawing functions now reliably return errors
-    as opposed to short successes.
+    as opposed to short successes. Dimensions of 0 to `ncplane_mergedown()`
+    now mean "everything".
   * `ncvisual_geom()` has been introduced, using the `ncvgeom` struct
     introduced for direct mode. This allows complete statement of geometry
     for an `ncvisual`. It replaces `ncvisual_blitter_geom()`, which has been
diff --git a/USAGE.md b/USAGE.md
index 19324847c..b60a813ba 100644
--- a/USAGE.md
+++ b/USAGE.md
@@ -918,12 +918,18 @@ struct ncplane* ncplane_dup(struct ncplane* n, void* opaque);
 
 // Merge the ncplane 'src' down onto the ncplane 'dst'. This is most rigorously
 // defined as "write to 'dst' the frame that would be rendered were the entire
-// stack made up only of 'src' and, below it, 'dst', and 'dst' was the entire
-// rendering region." Merging is independent of the position of 'src' viz 'dst'
-// on the z-axis. If 'src' does not intersect with 'dst', 'dst' will not be
-// changed, but it is not an error. The source plane still exists following
-// this operation. Do not supply the same plane for both 'src' and 'dst'.
-int ncplane_mergedown(struct ncplane* restrict src, struct ncplane* restrict dst);
+// stack made up only of the specified subregion of 'src' and, below it, the
+// subregion of 'dst' having the specified origin. Merging is independent of
+// the position of 'src' viz 'dst' on the z-axis. It is an error to define a
+// subregion that is not entirely contained within 'src'. It is an error to
+// define a target origin such that the projected subregion is not entirely
+// contained within 'dst'.  Behavior is undefined if 'src' and 'dst' are
+// equivalent. 'dst' is modified, but 'src' remains unchanged. Neither 'src'
+// nor 'dst' may have sprixels. Lengths of 0 mean "everything left".
+int ncplane_mergedown(struct ncplane* restrict src, struct ncplane* restrict dst,
+                      unsigned begsrcy, unsigned begsrcx,
+                      unsigned leny, unsigned lenx,
+                      unsigned dsty, unsigned dstx);
 
 // Merge the entirety of 'src' down onto the ncplane 'dst'. If 'src' does not
 // intersect with 'dst', 'dst' will not be changed, but it is not an error.
diff --git a/doc/man/man3/notcurses_plane.3.md b/doc/man/man3/notcurses_plane.3.md
index bc72b18e2..6cdf71f7b 100644
--- a/doc/man/man3/notcurses_plane.3.md
+++ b/doc/man/man3/notcurses_plane.3.md
@@ -205,7 +205,7 @@ typedef struct ncplane_options {
 
 **void notcurses_drop_planes(struct notcurses* ***nc***);**
 
-**int ncplane_mergedown(struct ncplane* ***src***, struct ncplane* ***dst***, int ***begsrcy***, int ***begsrcx***, int ***leny***, int ***lenx***, int ***dsty***, int ***dstx***);**
+**int ncplane_mergedown(struct ncplane* ***src***, struct ncplane* ***dst***, unsigned ***begsrcy***, unsigned ***begsrcx***, unsigned ***leny***, unsigned ***lenx***, unsigned ***dsty***, unsigned ***dstx***);**
 
 **int ncplane_mergedown_simple(struct ncplane* restrict ***src***, struct ncplane* restrict ***dst***);**
 
diff --git a/include/ncpp/Plane.hh b/include/ncpp/Plane.hh
index a988e5c5c..80bd1f9aa 100644
--- a/include/ncpp/Plane.hh
+++ b/include/ncpp/Plane.hh
@@ -452,17 +452,17 @@ namespace ncpp
 			return move_above (*above);
 		}
 
-		bool mergedown (Plane &dst, int begsrcy, int begsrcx, int leny, int lenx, int dsty, int dstx) const
+		bool mergedown (Plane &dst, unsigned begsrcy, unsigned begsrcx, unsigned leny, unsigned lenx, unsigned dsty, unsigned dstx) const
 		{
 			return mergedown (&dst, begsrcy, begsrcx, leny, lenx, dsty, dstx);
 		}
 
-		bool mergedown (Plane *dst, int begsrcy, int begsrcx, int leny, int lenx, int dsty, int dstx) const
+		bool mergedown (Plane *dst, unsigned begsrcy, unsigned begsrcx, unsigned leny, unsigned lenx, unsigned dsty, unsigned dstx) const
 		{
-			if (dst != nullptr && plane == dst->plane)
+			if (plane == dst->plane)
 				throw invalid_argument ("'dst' must refer to a different plane than the one this method is called on");
 
-			return error_guard (ncplane_mergedown (plane, dst != nullptr ? dst->plane : nullptr, begsrcy, begsrcx, leny, lenx, dsty, dstx), -1);
+			return error_guard (ncplane_mergedown (plane, dst->plane, begsrcy, begsrcx, leny, lenx, dsty, dstx), -1);
 		}
 
 		bool mergedown_simple (Plane &dst) const
@@ -472,7 +472,7 @@ namespace ncpp
 
 		bool mergedown_simple (Plane *dst) const
 		{
-			if (dst == nullptr || plane == dst->plane)
+			if (plane == dst->plane)
 				throw invalid_argument ("'dst' must refer to a different plane than the one this method is called on");
 
 			return error_guard (ncplane_mergedown_simple (plane, dst->plane), -1);
diff --git a/include/notcurses/notcurses.h b/include/notcurses/notcurses.h
index 1ab7417f5..18cec13c1 100644
--- a/include/notcurses/notcurses.h
+++ b/include/notcurses/notcurses.h
@@ -2302,28 +2302,32 @@ API int ncplane_stain(struct ncplane* n, int ystop, int xstop, uint64_t ul,
 // Merge the entirety of 'src' down onto the ncplane 'dst'. If 'src' does not
 // intersect with 'dst', 'dst' will not be changed, but it is not an error.
 API int ncplane_mergedown_simple(struct ncplane* RESTRICT src,
-                                 struct ncplane* RESTRICT dst);
+                                 struct ncplane* RESTRICT dst)
+  __attribute__ ((nonnull (1, 2)));
 
 // Merge the ncplane 'src' down onto the ncplane 'dst'. This is most rigorously
 // defined as "write to 'dst' the frame that would be rendered were the entire
 // stack made up only of the specified subregion of 'src' and, below it, the
 // subregion of 'dst' having the specified origin. Merging is independent of
 // the position of 'src' viz 'dst' on the z-axis. It is an error to define a
-// subregion of zero area, or that is not entirely contained within 'src'. It
-// is an error to define a target origin such that the projected subregion is
-// not entirely contained within 'dst'.  Behavior is undefined if 'src' and
-// 'dst' are equivalent. 'dst' is modified, but 'src' remains unchanged.
-// neither 'src' nor 'dst' may have sprixels.
+// subregion that is not entirely contained within 'src'. It is an error to
+// define a target origin such that the projected subregion is not entirely
+// contained within 'dst'.  Behavior is undefined if 'src' and 'dst' are
+// equivalent. 'dst' is modified, but 'src' remains unchanged. Neither 'src'
+// nor 'dst' may have sprixels. Lengths of 0 mean "everything left".
 API int ncplane_mergedown(struct ncplane* RESTRICT src,
                           struct ncplane* RESTRICT dst,
-                          int begsrcy, int begsrcx, int leny, int lenx,
-                          int dsty, int dstx);
+                          unsigned begsrcy, unsigned begsrcx,
+                          unsigned leny, unsigned lenx,
+                          unsigned dsty, unsigned dstx)
+  __attribute__ ((nonnull (1, 2)));
 
 // Erase every cell in the ncplane (each cell is initialized to the null glyph
 // and the default channels/styles). All cells associated with this ncplane are
 // invalidated, and must not be used after the call, *excluding* the base cell.
 // The cursor is homed. The plane's active attributes are unaffected.
-API void ncplane_erase(struct ncplane* n);
+API void ncplane_erase(struct ncplane* n)
+  __attribute__ ((nonnull (1)));
 
 // Erase every cell in the region starting at {ystart, xstart} and having size
 // {|ylen|x|xlen|} for non-zero lengths. If ystart and/or xstart are -1, the current
diff --git a/src/lib/render.c b/src/lib/render.c
index 479b20eb1..1dfaab899 100644
--- a/src/lib/render.c
+++ b/src/lib/render.c
@@ -526,31 +526,44 @@ postpaint(const tinfo* ti, nccell* lastframe, int dimy, int dimx,
 // merging one plane down onto another is basically just performing a render
 // using only these two planes, with the result written to the lower plane.
 int ncplane_mergedown(ncplane* restrict src, ncplane* restrict dst,
-                      int begsrcy, int begsrcx, int leny, int lenx,
-                      int dsty, int dstx){
+                      unsigned begsrcy, unsigned begsrcx,
+                      unsigned leny, unsigned lenx,
+                      unsigned dsty, unsigned dstx){
 //fprintf(stderr, "Merging down %d/%d @ %d/%d to %d/%d\n", leny, lenx, begsrcy, begsrcx, dsty, dstx);
-  if(dsty >= dst->leny || dstx >= dst->lenx){
-    logerror("Dest origin %d/%d ≥ dest dimensions %d/%d\n",
+  if(dsty >= (unsigned)dst->leny || dstx >= (unsigned)dst->lenx){
+    logerror("dest origin %u/%u ≥ dest dimensions %d/%d\n",
              dsty, dstx, dst->leny, dst->lenx);
     return -1;
   }
   if(dst->leny - leny < dsty || dst->lenx - lenx < dstx){
-    logerror("Dest len %d/%d ≥ dest dimensions %d/%d\n",
+    logerror("dest len %u/%u ≥ dest dimensions %d/%d\n",
              leny, lenx, dst->leny, dst->lenx);
     return -1;
   }
-  if(begsrcy >= src->leny || begsrcx >= src->lenx){
-    logerror("Source origin %d/%d ≥ source dimensions %d/%d\n",
+  if(begsrcy >= (unsigned)src->leny || begsrcx >= (unsigned)src->lenx){
+    logerror("source origin %u/%u ≥ source dimensions %d/%d\n",
              begsrcy, begsrcx, src->leny, src->lenx);
     return -1;
   }
+  if(leny == 0){
+    if((leny = src->leny - begsrcy) == 0){
+      logerror("source area was zero height\n");
+      return -1;
+    }
+  }
+  if(lenx == 0){
+    if((lenx = src->lenx - begsrcx) == 0){
+      logerror("source area was zero width\n");
+      return -1;
+    }
+  }
   if(src->leny - leny < begsrcy || src->lenx - lenx < begsrcx){
-    logerror("Source len %d/%d ≥ source dimensions %d/%d\n",
+    logerror("source len %u/%u ≥ source dimensions %d/%d\n",
              leny, lenx, src->leny, src->lenx);
     return -1;
   }
   if(src->sprite || dst->sprite){
-    logerror("Can't merge sprixel planes\n");
+    logerror("can't merge sprixel planes\n");
     return -1;
   }
   const int totalcells = dst->leny * dst->lenx;
@@ -558,7 +571,7 @@ int ncplane_mergedown(ncplane* restrict src, ncplane* restrict dst,
   const size_t crenderlen = sizeof(struct crender) * totalcells;
   struct crender* rvec = malloc(crenderlen);
   if(!rendfb || !rvec){
-    logerror("Error allocating render state for %dx%d\n", leny, lenx);
+    logerror("error allocating render state for %ux%u\n", leny, lenx);
     free(rendfb);
     free(rvec);
     return -1;
@@ -580,14 +593,7 @@ int ncplane_mergedown(ncplane* restrict src, ncplane* restrict dst,
 }
 
 int ncplane_mergedown_simple(ncplane* restrict src, ncplane* restrict dst){
-  // have to check dst, since we used to accept a NULL dst to mean the
-  // standard plane (this was unsafe, since src might be in another pile).
-  if(dst == NULL){
-    return -1;
-  }
-  int dimy, dimx;
-  ncplane_dim_yx(dst, &dimy, &dimx);
-  return ncplane_mergedown(src, dst, 0, 0, ncplane_dim_y(src), ncplane_dim_x(src), 0, 0);
+  return ncplane_mergedown(src, dst, 0, 0, 0, 0, 0, 0);
 }
 
 // write the nccell's UTF-8 extended grapheme cluster to the provided FILE*.
diff --git a/src/tetris/background.h b/src/tetris/background.h
index ec85179bf..650fd2bbd 100644
--- a/src/tetris/background.h
+++ b/src/tetris/background.h
@@ -18,7 +18,7 @@ void DrawBoard() { // draw all fixed components of the game
   stdplane_->get_dim(&y, &x);
   board_top_y_ = y - (BOARD_HEIGHT + 2);
   board_ = std::make_unique<ncpp::Plane>(BOARD_HEIGHT, BOARD_WIDTH * 2,
-                                          board_top_y_, x / 2 - (BOARD_WIDTH + 1));
+                                         board_top_y_, x / 2 - (BOARD_WIDTH + 1));
   uint64_t channels = 0;
   ncchannels_set_fg_rgb(&channels, 0x00b040);
   ncchannels_set_bg_alpha(&channels, NCALPHA_TRANSPARENT);