diff --git a/pkg/icingadb/entitiesbyid_test.go b/pkg/icingadb/entitiesbyid_test.go
new file mode 100644
index 000000000..ca2d0856a
--- /dev/null
+++ b/pkg/icingadb/entitiesbyid_test.go
@@ -0,0 +1,37 @@
+package icingadb
+
+import (
+	"github.com/icinga/icingadb/pkg/icingadb/v1"
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+func TestEntitiesById_Keys(t *testing.T) {
+	subtests := []struct {
+		name   string
+		input  EntitiesById
+		output []string
+	}{
+		{name: "nil"},
+		{
+			name:  "empty",
+			input: EntitiesById{},
+		},
+		{
+			name:   "one",
+			input:  EntitiesById{"one": nil},
+			output: []string{"one"},
+		},
+		{
+			name:   "two",
+			input:  EntitiesById{"one": nil, "two": &v1.EntityWithoutChecksum{}},
+			output: []string{"one", "two"},
+		},
+	}
+
+	for _, st := range subtests {
+		t.Run(st.name, func(t *testing.T) {
+			require.ElementsMatch(t, st.output, st.input.Keys())
+		})
+	}
+}