diff --git a/NEWS.md b/NEWS.md
index c6bcb50..b7731e9 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,10 @@
 # patchwork (development version)
 
+* Fixed `simplify_gt.gtable_patchwork()` indexing `widths` instead of
+  `heights` when computing the height of a nested patchwork, which threw
+  "index out of bounds" whenever every height in the nested layout was an
+  absolute unit
+
 # patchwork 1.3.2
 
 * Fixed a load-time bug that could throw spurious warnings
diff --git a/R/plot_patchwork.R b/R/plot_patchwork.R
index 1d4e180..5cbc3fe 100644
--- a/R/plot_patchwork.R
+++ b/R/plot_patchwork.R
@@ -404,7 +404,7 @@ simplify_gt.gtable_patchwork <- function(gt) {
     new_width <- unit(1, 'null')
   }
   if (all(is_abs_unit(gt$heights[panel_pos$t:panel_pos$b]))) {
-    new_height <- sum(convertHeight(gt$widths[panel_pos$t:panel_pos$b], 'mm'))
+    new_height <- sum(convertHeight(gt$heights[panel_pos$t:panel_pos$b], 'mm'))
   } else {
     new_height <- unit(1, 'null')
   }
diff --git a/tests/testthat/test-layout.R b/tests/testthat/test-layout.R
index 464a5b1..06a0d4c 100644
--- a/tests/testthat/test-layout.R
+++ b/tests/testthat/test-layout.R
@@ -101,3 +101,22 @@ test_that("various flavours of free() works", {
     free(free(p1 + p2 + p3 + p4, "label", "l"), "panel", "t") - p4 + p5 + plot_spacer()
   })
 })
+
+test_that("Nested patchworks with all-absolute-unit heights don't error", {
+  # simplify_gt.gtable_patchwork() used to index gt$widths instead of
+  # gt$heights when computing new_height, which threw "index out of bounds"
+  # as soon as a nested block's row count exceeded its column count - which
+  # happens whenever every height in the nested layout is an absolute unit
+  # (grid::unit(..., "cm")/"pt"/"in") rather than a relative one.
+  make_block <- function(n) {
+    wrap_elements(grid::textGrob(strrep("x\n", n))) + p1 +
+      plot_layout(
+        ncol = 1,
+        heights = grid::unit.c(grid::unit(n, "cm"), grid::unit(5, "cm"))
+      )
+  }
+
+  nested <- wrap_plots(list(make_block(1), make_block(3)), ncol = 1)
+
+  expect_no_error(patchworkGrob(nested))
+})