diff --git a/src/datasource/sql_series.ts b/src/datasource/sql_series.ts
index 2aca00276..2d5716c5b 100644
--- a/src/datasource/sql_series.ts
+++ b/src/datasource/sql_series.ts
@@ -293,6 +293,11 @@ export default class SqlSeries {
       if (keyColumns.length > 0) {
         metricKey = keyColumns.map((name: string) => {
           const value = row[name];
+
+          if (typeof value === 'undefined') {
+            return undefined;
+          }
+
           if (typeof value === 'object') {
             return JSON.stringify(value);
           } else {