--- a/mobile/android/mach_commands.py
+++ b/mobile/android/mach_commands.py
@@ -311,20 +311,30 @@ class MachCommands(MachCommandBase):
         # filter strings.xml, which is really UTF-8; the ellipsis character is
         # replaced with ??? in some encodings (including ASCII).  It's not yet
         # possible to filter with encodings in Gradle
         # (https://github.com/gradle/gradle/pull/520) and it's challenging to
         # do our filtering with Gradle's Ant support.  Moreover, all of the
         # Android tools expect UTF-8: see
         # http://tools.android.com/knownissues/encoding.  See
         # http://stackoverflow.com/a/21267635 for discussion of this approach.
+        #
+        # It's not even enough to set the encoding just for Gradle; it
+        # needs to be for JVMs spawned by Gradle as well.  This
+        # happens during the maven deployment generating the GeckoView
+        # documents; this works around "error: unmappable character
+        # for encoding ASCII" in exoplayer2.  See
+        # https://discuss.gradle.org/t/unmappable-character-for-encoding-ascii-when-building-a-utf-8-project/10692/11
+        # and especially https://stackoverflow.com/a/21755671.
+
         return self.run_process([self.substs['GRADLE']] + gradle_flags + ['--console=plain'] + args,
             append_env={
                 'GRADLE_OPTS': '-Dfile.encoding=utf-8',
                 'JAVA_HOME': java_home,
+                'JAVA_TOOL_OPTIONS': '-Dfile.encoding=utf-8',
             },
             pass_thru=True, # Allow user to run gradle interactively.
             ensure_exit_code=False, # Don't throw on non-zero exit code.
             cwd=mozpath.join(self.topsrcdir))
 
     @Command('gradle-install', category='devenv',
         conditions=[REMOVED])
     def gradle_install(self):