diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 43d8706255..ec68a55389 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -260,9 +260,9 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
       }
     }
     int flags = win_split_flags(fconfig.split, parent == NULL) | WSP_NOENTER;
+    int size = (flags & WSP_VERT) ? fconfig.width : fconfig.height;
 
     TRY_WRAP(err, {
-      int size = (flags & WSP_VERT) ? fconfig.width : fconfig.height;
       if (parent == NULL || parent == curwin) {
         wp = win_split_ins(size, flags, NULL, 0, NULL);
       } else {
@@ -277,12 +277,14 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
     });
     if (wp) {
       wp->w_config = fconfig;
-      // Without room for the requested size, window sizes may have been equalized instead.
-      // If the size differs from what was requested, try to set it again now.
-      if ((flags & WSP_VERT) && wp->w_width != fconfig.width) {
-        win_setwidth_win(fconfig.width, wp);
-      } else if (!(flags & WSP_VERT) && wp->w_height != fconfig.height) {
-        win_setheight_win(fconfig.height, wp);
+      if (size > 0) {
+        // Without room for the requested size, window sizes may have been equalized instead.
+        // If the size differs from what was requested, try to set it again now.
+        if ((flags & WSP_VERT) && wp->w_width != size) {
+          win_setwidth_win(size, wp);
+        } else if (!(flags & WSP_VERT) && wp->w_height != size) {
+          win_setheight_win(size, wp);
+        }
       }
     }
   } else {
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index e6d50b47f3..876b4a284c 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -2273,6 +2273,16 @@ describe('API/win', function()
       command('split')
       win = api.nvim_open_win(0, false, { win = 0, split = 'below', height = 10 })
       eq(10, api.nvim_win_get_height(win))
+
+      -- Still defaults to half-sized when no size was specified.
+      command('only')
+      eq(80, api.nvim_win_get_width(0))
+      api.nvim_open_win(0, true, { split = 'right' })
+      eq(40, api.nvim_win_get_width(0))
+
+      eq(22, api.nvim_win_get_height(0))
+      api.nvim_open_win(0, true, { split = 'below' })
+      eq(11, api.nvim_win_get_height(0))
     end)
   end)