diff --git a/rules/ast/ast.go b/rules/ast/ast.go
index a6f9e0195..7e891edd8 100644
--- a/rules/ast/ast.go
+++ b/rules/ast/ast.go
@@ -376,6 +376,7 @@ func (node *VectorAggregation) labelsToGroupingKey(labels clientmodel.Metric) ui
 	summer := fnv.New64a()
 	for _, label := range node.groupBy {
 		fmt.Fprint(summer, labels[label])
+		fmt.Fprint(summer, []byte{0})
 	}
 
 	return summer.Sum64()
diff --git a/rules/helpers_test.go b/rules/helpers_test.go
index ab1da04e8..867f66891 100644
--- a/rules/helpers_test.go
+++ b/rules/helpers_test.go
@@ -184,6 +184,27 @@ var testMatrix = ast.Matrix{
 		},
 		Values: append(getTestValueStream(0, 90, 10, testStartTime), getTestValueStream(0, 0, 10, testStartTime.Add(testSampleInterval*10))...),
 	},
+	// For label-key grouping regression test.
+	{
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "label_grouping_test",
+				"a": "aa",
+				"b": "bb",
+			},
+		},
+		Values: getTestValueStream(0, 100, 10, testStartTime),
+	},
+	{
+		Metric: clientmodel.COWMetric{
+			Metric: clientmodel.Metric{
+				clientmodel.MetricNameLabel: "label_grouping_test",
+				"a": "a",
+				"b": "abb",
+			},
+		},
+		Values: getTestValueStream(0, 200, 20, testStartTime),
+	},
 }
 
 var testVector = getTestVectorFromTestMatrix(testMatrix)
diff --git a/rules/rules_test.go b/rules/rules_test.go
index 2661e24ab..61f3b9c4e 100644
--- a/rules/rules_test.go
+++ b/rules/rules_test.go
@@ -553,6 +553,8 @@ func TestExpressions(t *testing.T) {
 				`testcounter_reset_end => 0 @[%v]`,
 				`testcounter_reset_middle => 50 @[%v]`,
 				`x{y="testvalue"} => 100 @[%v]`,
+				`label_grouping_test{a="a", b="abb"} => 200 @[%v]`,
+				`label_grouping_test{a="aa", b="bb"} => 100 @[%v]`,
 			},
 		},
 		{
@@ -641,6 +643,14 @@ func TestExpressions(t *testing.T) {
 			expr:       `sum(http_requests) offset 5m`,
 			shouldFail: true,
 		},
+		// Regression test for missing separator byte in labelsToGroupingKey.
+		{
+			expr: `sum(label_grouping_test) by (a, b)`,
+			output: []string{
+				`{a="a", b="abb"} => 200 @[%v]`,
+				`{a="aa", b="bb"} => 100 @[%v]`,
+			},
+		},
 	}
 
 	storage, closer := newTestStorage(t)