diff --git a/app/vmalert/datasource/type.go b/app/vmalert/datasource/type.go
index 10dfc0ca44..975a642e44 100644
--- a/app/vmalert/datasource/type.go
+++ b/app/vmalert/datasource/type.go
@@ -7,9 +7,6 @@ import (
 	"github.com/VictoriaMetrics/metricsql"
 )
 
-const graphiteType = "graphite"
-const prometheusType = "prometheus"
-
 // Type represents data source type
 type Type struct {
 	name string
@@ -17,12 +14,16 @@ type Type struct {
 
 // NewPrometheusType returns prometheus datasource type
 func NewPrometheusType() Type {
-	return Type{name: prometheusType}
+	return Type{
+		name: "prometheus",
+	}
 }
 
 // NewGraphiteType returns graphite datasource type
 func NewGraphiteType() Type {
-	return Type{name: graphiteType}
+	return Type{
+		name: "graphite",
+	}
 }
 
 // NewRawType returns datasource type from raw string
@@ -44,19 +45,19 @@ func (t *Type) Set(d Type) {
 // String implements String interface with default value.
 func (t Type) String() string {
 	if t.name == "" {
-		return prometheusType
+		return "prometheus"
 	}
 	return t.name
 }
 
 // ValidateExpr validates query expression with datasource ql.
 func (t *Type) ValidateExpr(expr string) error {
-	switch t.name {
-	case graphiteType:
+	switch t.String() {
+	case "graphite":
 		if _, err := graphiteql.Parse(expr); err != nil {
 			return fmt.Errorf("bad graphite expr: %q, err: %w", expr, err)
 		}
-	case "", prometheusType:
+	case "prometheus":
 		if _, err := metricsql.Parse(expr); err != nil {
 			return fmt.Errorf("bad prometheus expr: %q, err: %w", expr, err)
 		}
@@ -72,12 +73,13 @@ func (t *Type) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := unmarshal(&s); err != nil {
 		return err
 	}
+	if s == "" {
+		s = "prometheus"
+	}
 	switch s {
-	case "":
-		s = prometheusType
-	case graphiteType, prometheusType:
+	case "graphite", "prometheus":
 	default:
-		return fmt.Errorf("unknown datasource type=%q, want %q or %q", s, prometheusType, graphiteType)
+		return fmt.Errorf("unknown datasource type=%q, want %q or %q", s, "prometheus", "graphite")
 	}
 	t.name = s
 	return nil
diff --git a/app/vmalert/datasource/vm.go b/app/vmalert/datasource/vm.go
index 5d3dc62671..304d88cecd 100644
--- a/app/vmalert/datasource/vm.go
+++ b/app/vmalert/datasource/vm.go
@@ -80,10 +80,10 @@ func (s *VMStorage) Query(ctx context.Context, query string) ([]Metric, error) {
 	}
 
 	ts := time.Now()
-	switch s.dataSourceType.name {
-	case "", prometheusType:
+	switch s.dataSourceType.String() {
+	case "prometheus":
 		s.setPrometheusInstantReqParams(req, query, ts)
-	case graphiteType:
+	case "graphite":
 		s.setGraphiteReqParams(req, query, ts)
 	default:
 		return nil, fmt.Errorf("engine not found: %q", s.dataSourceType.name)
@@ -98,7 +98,7 @@ func (s *VMStorage) Query(ctx context.Context, query string) ([]Metric, error) {
 	}()
 
 	parseFn := parsePrometheusResponse
-	if s.dataSourceType.name != prometheusType {
+	if s.dataSourceType.name != "prometheus" {
 		parseFn = parseGraphiteResponse
 	}
 	return parseFn(req, resp)
@@ -108,7 +108,7 @@ func (s *VMStorage) Query(ctx context.Context, query string) ([]Metric, error) {
 // For Prometheus type see https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries
 // Graphite type isn't supported.
 func (s *VMStorage) QueryRange(ctx context.Context, query string, start, end time.Time) ([]Metric, error) {
-	if s.dataSourceType.name != prometheusType {
+	if s.dataSourceType.name != "prometheus" {
 		return nil, fmt.Errorf("%q is not supported for QueryRange", s.dataSourceType.name)
 	}
 	req, err := s.newRequestPOST()
diff --git a/app/vmalert/datasource/vm_test.go b/app/vmalert/datasource/vm_test.go
index 7dd584662b..946cedfe71 100644
--- a/app/vmalert/datasource/vm_test.go
+++ b/app/vmalert/datasource/vm_test.go
@@ -501,14 +501,14 @@ func TestRequestParams(t *testing.T) {
 			if err != nil {
 				t.Fatalf("unexpected error: %s", err)
 			}
-			switch tc.vm.dataSourceType.name {
-			case "", prometheusType:
+			switch tc.vm.dataSourceType.String() {
+			case "prometheus":
 				if tc.queryRange {
 					tc.vm.setPrometheusRangeReqParams(req, query, timestamp, timestamp)
 				} else {
 					tc.vm.setPrometheusInstantReqParams(req, query, timestamp)
 				}
-			case graphiteType:
+			case "graphite":
 				tc.vm.setGraphiteReqParams(req, query, timestamp)
 			}
 			tc.checkFn(t, req)