diff --git a/x-pack/plugins/code/server/utils/composite_source_merger.test.ts b/x-pack/plugins/code/server/utils/composite_source_merger.test.ts
index ea774c1de97bc..91adfd6372e0e 100644
--- a/x-pack/plugins/code/server/utils/composite_source_merger.test.ts
+++ b/x-pack/plugins/code/server/utils/composite_source_merger.test.ts
@@ -39,4 +39,5 @@ test('extract source by ranges', () => {
   const expectedLineNumbers = ['1', '2', '3', '...', '8', '9', '10', '11'];
   expect(lineMappings.toStringArray('...')).toEqual(expectedLineNumbers);
   expect(lineMappings.lineNumber(7)).toBe(5);
+  expect(lineMappings.lineNumber(0)).toBe(1);
 });
diff --git a/x-pack/plugins/code/server/utils/composite_source_merger.ts b/x-pack/plugins/code/server/utils/composite_source_merger.ts
index 9e7dfbb05550c..982e77a28cd93 100644
--- a/x-pack/plugins/code/server/utils/composite_source_merger.ts
+++ b/x-pack/plugins/code/server/utils/composite_source_merger.ts
@@ -119,8 +119,12 @@ export class LineMapping {
   }
 
   public lineNumber(originLineNumber: number, startAtLine: number = 1) {
-    const n = this.reverseMap.get(originLineNumber) || Number.NaN;
-    return n + startAtLine;
+    const n = this.reverseMap.get(originLineNumber);
+    if (n === undefined) {
+      return Number.NaN;
+    } else {
+      return n + startAtLine;
+    }
   }
 
   public hasLine(line: number) {