diff --git a/changelogs/unreleased/42161-gitaly-commitservice-encoding-undefinedconversionerror-u-c124-from-utf-8-to-ascii-8bit.yml b/changelogs/unreleased/42161-gitaly-commitservice-encoding-undefinedconversionerror-u-c124-from-utf-8-to-ascii-8bit.yml
new file mode 100644
index 00000000000..c64bee9126e
--- /dev/null
+++ b/changelogs/unreleased/42161-gitaly-commitservice-encoding-undefinedconversionerror-u-c124-from-utf-8-to-ascii-8bit.yml
@@ -0,0 +1,5 @@
+---
+title: Fix encoding issue when counting commit count
+merge_request: 16637
+author:
+type: fixed
diff --git a/lib/gitlab/gitaly_client/commit_service.rb b/lib/gitlab/gitaly_client/commit_service.rb
index 2231371cfa7..33a8d3e5612 100644
--- a/lib/gitlab/gitaly_client/commit_service.rb
+++ b/lib/gitlab/gitaly_client/commit_service.rb
@@ -125,11 +125,11 @@ module Gitlab
       def commit_count(ref, options = {})
         request = Gitaly::CountCommitsRequest.new(
           repository: @gitaly_repo,
-          revision: ref
+          revision: encode_binary(ref)
         )
         request.after = Google::Protobuf::Timestamp.new(seconds: options[:after].to_i) if options[:after].present?
         request.before = Google::Protobuf::Timestamp.new(seconds: options[:before].to_i) if options[:before].present?
-        request.path = options[:path] if options[:path].present?
+        request.path = encode_binary(options[:path]) if options[:path].present?
         request.max_count = options[:max_count] if options[:max_count].present?
 
         GitalyClient.call(@repository.storage, :commit_service, :count_commits, request, timeout: GitalyClient.medium_timeout).count
diff --git a/spec/lib/gitlab/gitaly_client/commit_service_spec.rb b/spec/lib/gitlab/gitaly_client/commit_service_spec.rb
index b2275119a04..3722a91c050 100644
--- a/spec/lib/gitlab/gitaly_client/commit_service_spec.rb
+++ b/spec/lib/gitlab/gitaly_client/commit_service_spec.rb
@@ -131,6 +131,29 @@ describe Gitlab::GitalyClient::CommitService do
     end
   end
 
+  describe '#commit_count' do
+    before do
+      expect_any_instance_of(Gitaly::CommitService::Stub)
+        .to receive(:count_commits)
+        .with(gitaly_request_with_path(storage_name, relative_path),
+              kind_of(Hash))
+        .and_return([])
+    end
+
+    it 'sends a commit_count message' do
+      client.commit_count(revision)
+    end
+
+    context 'with UTF-8 params strings' do
+      let(:revision) { "branch\u011F" }
+      let(:path) { "foo/\u011F.txt" }
+
+      it 'handles string encodings correctly' do
+        client.commit_count(revision, path: path)
+      end
+    end
+  end
+
   describe '#find_commit' do
     let(:revision) { '4b825dc642cb6eb9a060e54bf8d69288fbee4904' }
     it 'sends an RPC request' do