@@ -49,11 +49,9 @@ export class SourceFile {
     const sources: SourceFile[] = [];
     const names: string[] = [];
 
-    // Ensure a mapping line array for each line in the generated source.
-    const mappings: SourceMapMappings = this.lineLengths.map(() => []);
+    const mappings: SourceMapMappings = [];
 
     for (const mapping of this.flattenedMappings) {
-      const mappingLine = mappings[mapping.generatedSegment.line];
       const sourceIndex = findIndexOrAdd(sources, mapping.originalSource);
       const mappingArray: SourceMapSegment = [
         mapping.generatedSegment.column,
@@ -65,7 +63,14 @@ export class SourceFile {
         const nameIndex = findIndexOrAdd(names, mapping.name);
         mappingArray.push(nameIndex);
       }
-      mappingLine.push(mappingArray);
+
+      // Ensure a mapping line array for this mapping.
+      const line = mapping.generatedSegment.line;
+      while (line >= mappings.length) {
+        mappings.push([]);
+      }
+      // Add this mapping to the line
+      mappings[line].push(mappingArray);
     }
 
     const sourcePathDir = dirname(this.sourcePath);
 
@@ -174,7 +174,6 @@ runInEachFileSystem(() => {
           [5, 0, 1, 20], [7, 0, 1, 22], [12, 0, 1, 27], [14, 0, 1, 28], [15, 0, 1, 29],
           [9, 0, 2, 13], [10, 0, 2, 14]
         ],
-        [],
       ];
 
       JS_CONTENT_MAP = fromObject({
@@ -196,15 +195,12 @@ runInEachFileSystem(() => {
         'file': 'file.js',
         'sources': ['file.js'],
         'names': [],
-        'mappings': encode([
-          [], [], [], [], [], [], [], [], [], [], [], [], [[0, 0, 0, 0]],
-          [], [], [], [], [], [], [], [], []
-        ]),
+        'mappings': encode([[], [], [], [], [], [], [], [], [], [], [], [], [[0, 0, 0, 0]]]),
         'sourcesContent': [JS_CONTENT.contents],
       });
 
       const MERGED_OUTPUT_PROGRAM_MAPPINGS: SourceMapMappings =
-          [[], [], [], [], [], [], [], [], [], [], [], [], ...JS_CONTENT_MAPPINGS, []];
+          [[], [], [], [], [], [], [], [], [], [], [], [], ...JS_CONTENT_MAPPINGS];
 
       MERGED_OUTPUT_PROGRAM_MAP = fromObject({
         'version': 3,
 
@@ -229,6 +229,32 @@ runInEachFileSystem(() => {
             [[1, 0, 0, 0], [2, 0, 0, 2], [3, 0, 0, 3], [3, 0, 0, 6], [4, 0, 0, 1], [5, 0, 0, 7]]
           ]));
         });
+
+        it('should handle mappings that map from lines outside of the actual content lines', () => {
+          const bSource = new SourceFile(_('/foo/src/b.js'), 'abcdef', null, false, []);
+          const aToBSourceMap: RawSourceMap = {
+            mappings: encode([
+              [[0, 0, 0, 0], [2, 0, 0, 3], [4, 0, 0, 2], [5, 0, 0, 5]],
+              [
+                [0, 0, 0, 0],  // Extra mapping from a non-existent line
+              ]
+            ]),
+            names: [],
+            sources: ['b.js'],
+            version: 3
+          };
+          const aSource =
+              new SourceFile(_('/foo/src/a.js'), 'abdecf', aToBSourceMap, false, [bSource]);
+
+          const aTocSourceMap = aSource.renderFlattenedSourceMap();
+          expect(aTocSourceMap.version).toEqual(3);
+          expect(aTocSourceMap.file).toEqual('a.js');
+          expect(aTocSourceMap.names).toEqual([]);
+          expect(aTocSourceMap.sourceRoot).toBeUndefined();
+          expect(aTocSourceMap.sources).toEqual(['b.js']);
+          expect(aTocSourceMap.sourcesContent).toEqual(['abcdef']);
+          expect(aTocSourceMap.mappings).toEqual(aToBSourceMap.mappings);
+        });
       });
     });