diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 62aba51afec..395f6448890 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -4391,7 +4391,7 @@ namespace ts {
                 return;
             }
 
-            const { line: sourceLine, character: sourceCharacter } = getLineAndCharacterOfPosition(currentSourceFile!, pos);
+            const { line: sourceLine, character: sourceCharacter } = getLineAndCharacterOfPosition(sourceMapSource, pos);
             sourceMapGenerator!.addMapping(
                 writer.getLine(),
                 writer.getColumn(),
diff --git a/src/testRunner/unittests/customTransforms.ts b/src/testRunner/unittests/customTransforms.ts
index 304c0a55c59..aef9ed745ec 100644
--- a/src/testRunner/unittests/customTransforms.ts
+++ b/src/testRunner/unittests/customTransforms.ts
@@ -95,6 +95,39 @@ namespace ts {
             module: ModuleKind.ES2015,
             emitDecoratorMetadata: true,
             experimentalDecorators: true
-         });
+        });
+
+        emitsCorrectly("sourceMapExternalSourceFiles",
+            [
+                {
+                    file: "source.ts",
+                    // The text of length 'changed' is made to be on two lines so we know the line map change
+                    text: `\`multi
+                    line\`
+'change'`
+                },
+            ],
+            {
+                before: [
+                    context => node => visitNode(node, function visitor(node: Node): Node {
+                        if (isStringLiteral(node) && node.text === "change") {
+                            const text = "'changed'";
+                            const lineMap = computeLineStarts(text);
+                            setSourceMapRange(node, {
+                                pos: 0, end: text.length, source: {
+                                    text,
+                                    fileName: "another.html",
+                                    lineMap,
+                                    getLineAndCharacterOfPosition: pos => computeLineAndCharacterOfPosition(lineMap, pos)
+                                }
+                            });
+                            return node;
+                        }
+                        return visitEachChild(node, visitor, context);
+                    })
+                ]
+            },
+            { sourceMap: true }
+        );
     });
 }
diff --git a/tests/baselines/reference/customTransforms/sourceMapExternalSourceFiles.js b/tests/baselines/reference/customTransforms/sourceMapExternalSourceFiles.js
new file mode 100644
index 00000000000..7191d615fc6
--- /dev/null
+++ b/tests/baselines/reference/customTransforms/sourceMapExternalSourceFiles.js
@@ -0,0 +1,7 @@
+// [source.js.map]
+{"version":3,"file":"source.js","sourceRoot":"","sources":["source.ts","another.html"],"names":[],"mappings":"AAAA,iCACyB,CAAA;ACDzB,QAAS,CDED"}
+
+// [source.js]
+"multi\n                    line";
+'change';
+//# sourceMappingURL=source.js.map
\ No newline at end of file