diff --git a/NEWS.md b/NEWS.md
index b176ff66..f4bd4d73 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,6 @@
 # thematic 0.1.1.9000
 
+* Closed #90: Enabling thematic no longer fails when used with package(s) that resgiter `{ggplot2}` methods via `registerS3method()` within `.onLoad()` (`{zoo}` is one such package that does this). (#90)
 * Closed #86: thematic_rmd() now works as expected in `shiny: runtime` Rmd documents (#87)
 * Closed #82: Fixed a bug with element_blank() in plot-specific user code not being respected by thematic. (#83) 
 
diff --git a/R/ggplot.R b/R/ggplot.R
index 1512ad0b..e40c9bc4 100644
--- a/R/ggplot.R
+++ b/R/ggplot.R
@@ -18,6 +18,7 @@ ggplot_build_set <- function() {
   ggplot_build <- getFromNamespace("ggplot_build", "ggplot2")
   .globals$ggplot_build <- getFromNamespace("ggplot_build.ggplot", "ggplot2")
   assign_in_namespace <- assignInNamespace
+  ensure_s3_methods_matrix()
   assign_in_namespace("ggplot_build.ggplot", ggthematic_build, "ggplot2")
 }
 
@@ -25,11 +26,24 @@ ggplot_build_restore <- function() {
   if (is.function(.globals$ggplot_build)) {
     ggplot_build <- getFromNamespace("ggplot_build", "ggplot2")
     assign_in_namespace <- assignInNamespace
+    ensure_s3_methods_matrix()
     assign_in_namespace("ggplot_build.ggplot", .globals$ggplot_build, "ggplot2")
     rm("ggplot_build", envir = .globals)
   }
 }
 
+# When registerS3method() is called in an onLoad() hook, it causes
+# assignInNamespace() to no longer work for S3 methods because this
+# S3methods namespace info gets coerced into a list
+# (which assignInNamespace() isn't expecting). Thus, here, we make
+# sure that the info is a matrix before calling assignInNamespace()
+# https://github.com/rstudio/thematic/issues/90#issuecomment-780224962
+ensure_s3_methods_matrix <- function(pkg = "ggplot2") {
+  S3 <- .getNamespaceInfo(asNamespace(pkg), "S3methods")
+  S3 <- matrix(as.character(S3), nrow = nrow(S3), ncol = ncol(S3))
+  setNamespaceInfo(asNamespace(pkg), "S3methods", S3)
+}
+
 # N.B. If you make changes here, plotly might have to as well!
 # https://github.com/ropensci/plotly/pull/1801/files#diff-3afd3a8e6a2cbc84a7afc6d2d06ec5e3R429
 ggthematic_build <- function(p, ggplot_build = NULL, theme = NULL) {
diff --git a/tests/testthat/agg_png/tests/shinytest/mytest-expected-mac/001.png b/tests/testthat/agg_png/tests/shinytest/mytest-expected-mac/001.png
index 8f618815..52e03895 100644
Binary files a/tests/testthat/agg_png/tests/shinytest/mytest-expected-mac/001.png and b/tests/testthat/agg_png/tests/shinytest/mytest-expected-mac/001.png differ
diff --git a/tests/testthat/agg_png/tests/shinytest/mytest-expected-win/001.png b/tests/testthat/agg_png/tests/shinytest/mytest-expected-win/001.png
index caa0b08f..af623ab3 100644
Binary files a/tests/testthat/agg_png/tests/shinytest/mytest-expected-win/001.png and b/tests/testthat/agg_png/tests/shinytest/mytest-expected-win/001.png differ
diff --git a/tests/testthat/shiny_runtime/tests/shinytest/mytest-expected-win/001.json b/tests/testthat/shiny_runtime/tests/shinytest/mytest-expected-win/001.json
index 3ff9c91b..4913abeb 100644
--- a/tests/testthat/shiny_runtime/tests/shinytest/mytest-expected-win/001.json
+++ b/tests/testthat/shiny_runtime/tests/shinytest/mytest-expected-win/001.json
@@ -4,7 +4,7 @@
   },
   "output": {
     "__reactivedoc__": {
-      "html": "[html data sha1: bba70ec94501a4898cb719daf599f07ff70cd290]",
+      "html": "[html data sha1: af3206cd4165f61da1bc6a760fc097ceb10273e5]",
       "deps": [
         {
           "name": "jquery",