diff --git a/src/source-map-tree.ts b/src/source-map-tree.ts
index ae0687f..38bd3d5 100644
--- a/src/source-map-tree.ts
+++ b/src/source-map-tree.ts
@@ -75,9 +75,7 @@ export default class SourceMapTree {
     let lastSegmentsLine = -1;
     for (let i = 0; i < rootMappings.length; i++) {
       const segments = rootMappings[i];
-      const traced =
-        traceUneditedLine(segments, rootSources, intoSegments) ||
-        traceLine(segments, rootSources, rootNames, intoSegments);
+      const traced = traceLine(segments, rootSources, rootNames, intoSegments);
       if (traced.length > 0) lastSegmentsLine = i;
       mappings.push(traced);
     }
@@ -105,10 +103,9 @@ export default class SourceMapTree {
   ): SourceMapSegment[] {
     if (line >= this.lines) return [];
 
-    const { sources } = this;
     const { mappings, names } = this.map;
     const segments = mappings[line];
-    return traceUneditedLine(segments, sources, into) || traceLine(segments, sources, names, into);
+    return traceLine(segments, this.sources, names, into);
   }
 
   /**
@@ -174,9 +171,21 @@ export default class SourceMapTree {
   }
 }
 
-// An unedited line either contains no segments, or it contains only a line marker segment.
-// Line markers are the default "lowres" segment generated by magic-string, and match [0, SOURCE, LINE, 0].
-// For these markers, we trace directly from the referenced source instead of from this map.
+function traceLine(
+  segments: SourceMapSegment[],
+  sources: Source[],
+  names: string[],
+  into: (s: SourceMapSegmentObject) => SourceMapSegment
+): SourceMapSegment[] {
+  return (
+    traceUneditedLine(segments, sources, into) || traceEditedLine(segments, sources, names, into)
+  );
+}
+
+// An unedited line either contains only a line marker segment. Line markers are
+// the default "lowres" segment generated by magic-string, and match [0, SOURCE,
+// LINE, 0]. For these markers, we inherit the mappings from the referenced
+// source directly, instead of trying to match the segments.
 function traceUneditedLine(
   segments: SourceMapSegment[],
   sources: Source[],
@@ -194,7 +203,7 @@ function traceUneditedLine(
   return sources[segment[1]].traceLine(segment[2], into);
 }
 
-function traceLine(
+function traceEditedLine(
   segments: SourceMapSegment[],
   sources: Source[],
   names: string[],
diff --git a/test/unit/source-map-tree.ts b/test/unit/source-map-tree.ts
index c72d5ec..3b89b6b 100644
--- a/test/unit/source-map-tree.ts
+++ b/test/unit/source-map-tree.ts
@@ -16,7 +16,7 @@
 
 import OriginalSource from '../../src/original-source';
 import SourceMapTree from '../../src/source-map-tree';
-import type { DecodedSourceMap } from '../../src/types';
+import type { SourceMapSegment, SourceMapSegmentObject, DecodedSourceMap } from '../../src/types';
 
 describe('SourceMapTree', () => {
   describe('traceMappings()', () => {
@@ -215,6 +215,100 @@ describe('SourceMapTree', () => {
         });
       });
     });
+
+    describe('unedited line', () => {
+      it('inherits mappings from child map', () => {
+        const map: DecodedSourceMap = {
+          ...baseMap,
+          mappings: [[[0, 0, 0, 0]]],
+        };
+
+        const source = new SourceMapTree(map, [child]);
+        const traced = source.traceMappings();
+
+        expect(traced).toMatchObject({
+          mappings: [
+            [
+              [0, 0, 0, 0],
+              [4, 0, 1, 1],
+            ],
+          ],
+        });
+      });
+
+      it('inherits mappings from any child map', () => {
+        const map: DecodedSourceMap = {
+          ...baseMap,
+          mappings: [[[0, 1, 0, 0]]],
+        };
+
+        const source = new SourceMapTree(map, [new OriginalSource('foo.js', ''), child]);
+        const traced = source.traceMappings();
+
+        expect(traced).toMatchObject({
+          sources: [`${sourceRoot}/original.js`],
+          mappings: [
+            [
+              [0, 0, 0, 0],
+              [4, 0, 1, 1],
+            ],
+          ],
+        });
+      });
+    });
+  });
+
+  describe('traceLine()', () => {
+    const map: DecodedSourceMap = {
+      mappings: [
+        [[0, 0, 0, 0]],
+        [
+          [0, 0, 0, 0],
+          [1, 0, 1, 2],
+        ],
+      ],
+      names: [],
+      sources: ['middle.js'],
+      version: 3,
+    };
+    const middle: DecodedSourceMap = {
+      mappings: [
+        [
+          [0, 0, 0, 0],
+          [1, 0, 0, 0],
+          [2, 0, 0, 0],
+          [4, 0, 1, 1],
+        ], // line 0
+        [[2, 0, 1, 1]], // line 1 - maps to line 0 col 0
+      ],
+      names: ['name'],
+      sources: ['child.js'],
+      version: 3,
+    };
+    const source = new SourceMapTree(map, [
+      new SourceMapTree(middle, [new OriginalSource('child.js', '')]),
+    ]);
+    function into(s: SourceMapSegmentObject): SourceMapSegment {
+      return [s.outputColumn, 0, s.line, s.column];
+    }
+
+    it('inherits segments from unedited line', () => {
+      const traced = source.traceLine(0, into);
+
+      expect(traced).toEqual([
+        [0, 0, 0, 0],
+        [4, 0, 1, 1],
+      ]);
+    });
+
+    it('traces segments from an edited line', () => {
+      const traced = source.traceLine(1, into);
+
+      expect(traced).toEqual([
+        [0, 0, 0, 0],
+        [1, 0, 1, 1],
+      ]);
+    });
   });
 
   describe('traceSegment()', () => {