diff --git a/test/engine/imap-db/imap-db-database-test.vala b/test/engine/imap-db/imap-db-database-test.vala
index 44a4845f..5b758c2a 100644
--- a/test/engine/imap-db/imap-db-database-test.vala
+++ b/test/engine/imap-db/imap-db-database-test.vala
@@ -147,7 +147,18 @@ class Geary.ImapDB.DatabaseTest : TestCase {
             INSERT INTO Test (test_str) VALUES ('BB');
             INSERT INTO Test (test_str) VALUES ('🤯');
         """);
+
         string[] expected = { "a", "BB", "B", "🤯" };
+        // Distros don't ship well-known locales other than C.UTF-8 by
+        // default, but Flatpak does not currently ship C.UTF-8 at
+        // all, so need to support both. :(
+        //
+        // See test-engine.vala and
+        // https://gitlab.com/freedesktop-sdk/freedesktop-sdk/issues/812
+        if (GLib.Intl.setlocale(LocaleCategory.COLLATE, null) != "C.UTF-8") {
+            // en_US.UTF-8:
+            expected = { "BB", "B", "a", "🤯" };
+        }
 
         Db.Result result = db.query(
             "SELECT test_str FROM Test ORDER BY test_str COLLATE UTF8COLL DESC"
diff --git a/test/test-engine.vala b/test/test-engine.vala
index 55862bd5..a972d75e 100644
--- a/test/test-engine.vala
+++ b/test/test-engine.vala
@@ -10,11 +10,19 @@ int main(string[] args) {
      * Initialise all the things.
      */
 
-    Test.init(ref args);
-
     // Ensure things like e.g. GLib's formatting routines uses a
-    // UTF-8-based locale rather ASCII
-    GLib.Intl.setlocale(LocaleCategory.ALL, "C.UTF-8");
+    // well-known UTF-8-based locale rather ASCII.
+    //
+    // Distros don't ship well-known locales other than C.UTF-8 by
+    // default, but Flatpak does not currently support C.UTF-8, so
+    // need to try both. :(
+    // https://gitlab.com/freedesktop-sdk/freedesktop-sdk/issues/812
+    string? locale = GLib.Intl.setlocale(LocaleCategory.ALL, "C.UTF-8");
+    if (locale == null) {
+        GLib.Intl.setlocale(LocaleCategory.ALL, "en_US.UTF-8");
+    }
+
+    Test.init(ref args);
 
     Geary.RFC822.init();
     Geary.HTML.init();