diff --git a/app/models/ci/build.rb b/app/models/ci/build.rb
index ee987949080..b230b7f47ef 100644
--- a/app/models/ci/build.rb
+++ b/app/models/ci/build.rb
@@ -467,7 +467,7 @@ module Ci
 
       if cache && project.jobs_cache_index
         cache = cache.merge(
-          key: "#{cache[:key]}_#{project.jobs_cache_index}")
+          key: "#{cache[:key]}-#{project.jobs_cache_index}")
       end
 
       [cache]
diff --git a/changelogs/unreleased/43373-fix-cache-index-appending.yml b/changelogs/unreleased/43373-fix-cache-index-appending.yml
new file mode 100644
index 00000000000..fdb293ea04d
--- /dev/null
+++ b/changelogs/unreleased/43373-fix-cache-index-appending.yml
@@ -0,0 +1,5 @@
+---
+title: Fix issue with cache key being empty when variable used as the key
+merge_request: 17260
+author:
+type: fixed
diff --git a/spec/models/ci/build_spec.rb b/spec/models/ci/build_spec.rb
index 2b6b6a61182..c27313ed88b 100644
--- a/spec/models/ci/build_spec.rb
+++ b/spec/models/ci/build_spec.rb
@@ -277,7 +277,7 @@ describe Ci::Build do
           allow_any_instance_of(Project).to receive(:jobs_cache_index).and_return(1)
         end
 
-        it { is_expected.to be_an(Array).and all(include(key: "key_1")) }
+        it { is_expected.to be_an(Array).and all(include(key: "key-1")) }
       end
 
       context 'when project does not have jobs_cache_index' do