diff --git a/pkg/api/api_test.go b/pkg/api/api_test.go
index 0948884..e85eb31 100644
--- a/pkg/api/api_test.go
+++ b/pkg/api/api_test.go
@@ -50,6 +50,8 @@ func jpfEcho(arguments []interface{}) (interface{}, error) {
 }
 
 func TestSearch(t *testing.T) {
+	type Label string
+
 	type args struct {
 		expression string
 		data       interface{}
@@ -145,6 +147,38 @@ func TestSearch(t *testing.T) {
 			data:       []interface{}{map[string]any{}, nil, map[string]any{"foo": "bar"}},
 		},
 		want: true,
+	}, {
+		args: args{
+			expression: "length(@[?metric.__name__ == 'foo'])",
+			data: []struct {
+				Metric map[Label]any
+			}{{
+				Metric: map[Label]any{
+					"__name__": "foo",
+				},
+			}, {
+				Metric: map[Label]any{
+					"__name__": "bar",
+				},
+			}},
+		},
+		want: 1.0,
+	}, {
+		args: args{
+			expression: "length(@[?metric.__name__ == 'foo'])",
+			data: []struct {
+				Metric map[string]string
+			}{{
+				Metric: map[string]string{
+					"__name__": "foo",
+				},
+			}, {
+				Metric: map[string]string{
+					"__name__": "bar",
+				},
+			}},
+		},
+		want: 1.0,
 	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
diff --git a/pkg/interpreter/interpreter.go b/pkg/interpreter/interpreter.go
index eb3c437..0cd0c19 100644
--- a/pkg/interpreter/interpreter.go
+++ b/pkg/interpreter/interpreter.go
@@ -154,19 +154,7 @@ func (intr *treeInterpreter) execute(node parsing.ASTNode, value interface{}, fu
 		}
 		return functionCaller.CallFunction(node.Value.(string), resolvedArgs)
 	case parsing.ASTField:
-		key := node.Value.(string)
-		var result interface{}
-		if m, ok := value.(map[string]interface{}); ok {
-			result = m[key]
-			if result != nil {
-				return result, nil
-			}
-		}
-		result = intr.fieldFromStruct(node.Value.(string), value)
-		if result != nil {
-			return result, nil
-		}
-		return nil, nil
+		return extractField(value, node.Value.(string))
 	case parsing.ASTFilterProjection:
 		left, err := intr.execute(node.Children[0], value, functionCaller)
 		if err != nil {
@@ -462,29 +450,40 @@ func (intr *treeInterpreter) execute(node parsing.ASTNode, value interface{}, fu
 	return nil, errors.New("Unknown AST node: " + node.NodeType.String())
 }
 
-func (intr *treeInterpreter) fieldFromStruct(key string, value interface{}) interface{} {
-	rv := reflect.ValueOf(value)
-	first, n := utf8.DecodeRuneInString(key)
-	fieldName := string(unicode.ToUpper(first)) + key[n:]
-	if rv.Kind() == reflect.Struct {
-		v := rv.FieldByName(fieldName)
-		if !v.IsValid() {
-			return nil
-		}
-		return v.Interface()
-	} else if rv.Kind() == reflect.Ptr {
-		// Handle multiple levels of indirection?
-		if rv.IsNil() {
-			return nil
-		}
-		rv = rv.Elem()
-		v := rv.FieldByName(fieldName)
-		if !v.IsValid() {
-			return nil
-		}
-		return v.Interface()
+func extractField(value any, field string) (any, error) {
+	if value == nil {
+		return nil, nil
+	}
+	if m, ok := value.(map[string]interface{}); ok {
+		return m[field], nil
+	}
+	return extractFieldUsingReflection(reflect.ValueOf(value), field)
+}
+
+func extractFieldUsingReflection(value reflect.Value, field string) (any, error) {
+	if value.Kind() == reflect.Ptr {
+		if value.IsNil() {
+			return nil, nil
+		}
+		return extractFieldUsingReflection(value.Elem(), field)
+	} else if value.Kind() == reflect.Struct {
+		first, n := utf8.DecodeRuneInString(field)
+		fieldName := string(unicode.ToUpper(first)) + field[n:]
+		value := value.FieldByName(fieldName)
+		if value.IsValid() {
+			return value.Interface(), nil
+		}
+	} else if value.Kind() == reflect.Map {
+		keyType := value.Type().Key()
+		if reflect.TypeOf(field).ConvertibleTo(keyType) {
+			key := reflect.ValueOf(field)
+			value := value.MapIndex(key.Convert(keyType))
+			if value.IsValid() {
+				return value.Interface(), nil
+			}
+		}
 	}
-	return nil
+	return nil, nil
 }
 
 func (intr *treeInterpreter) flattenWithReflection(value interface{}) (interface{}, error) {