diff --git a/Sources/SwiftDriver/IncrementalCompilation/DependencyKey.swift b/Sources/SwiftDriver/IncrementalCompilation/DependencyKey.swift
index 77e531515..fc5478a30 100644
--- a/Sources/SwiftDriver/IncrementalCompilation/DependencyKey.swift
+++ b/Sources/SwiftDriver/IncrementalCompilation/DependencyKey.swift
@@ -19,6 +19,10 @@ import TSCBasic
     try? fileSystem.getFileInfo(file).modTime
   }
 
+  var swiftModuleFile: TypedVirtualPath? {
+    isSwiftModule ? TypedVirtualPath(file: file, type: .swiftModule) : nil
+  }
+
   public var description: String {
     file.name
   }
@@ -39,8 +43,13 @@ public struct FingerprintedExternalDependency: Hashable, Equatable, ExternalDepe
     assert(verifyExternalDependencyAndFingerprint())
   }
   var externalDependencyToCheck: ExternalDependency? { externalDependency }
-  var isIncremental: Bool {
-    fingerprint != nil && externalDependency.isSwiftModule
+  var incrementalDependencySource: DependencySource? {
+    guard let _ = fingerprint,
+          let swiftModuleFile = externalDependency.swiftModuleFile
+    else {
+      return nil
+    }
+    return DependencySource(swiftModuleFile)
   }
 }
 
diff --git a/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift b/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift
index f1a6bfb8d..8708b9c59 100644
--- a/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift
+++ b/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift
@@ -209,12 +209,9 @@ extension ModuleDependencyGraph {
     externalDependency: FingerprintedExternalDependency,
     fileSystem: FileSystem
   ) -> Integrator.Results? {
-    // Save time; don't even try
-    guard externalDependency.isIncremental else {
+    guard let dependencySource = externalDependency.incrementalDependencySource else {
       return Integrator.Results()
     }
-    let file = externalDependency.externalDependency.file
-    let dependencySource = DependencySource(file)
     reporter?.report("integrating incremental external dependency",
                      dependencySource.typedFile)
     guard let sourceGraph = dependencySource.read(
@@ -445,6 +442,7 @@ extension ModuleDependencyGraph {
     case unexpectedSubblock
     case bogusNameOrContext
     case unknownKind
+    case unknownDependencySourceExtension
   }
 
   /// Attempts to read a serialized dependency graph from the given path.
@@ -559,9 +557,12 @@ extension ModuleDependencyGraph {
           let swiftDepsStr = hasSwiftDeps ? identifiers[Int(record.fields[5])] : nil
           let hasFingerprint = Int(record.fields[6]) != 0
           let fingerprint = hasFingerprint ? fingerprintStr : nil
-          let dependencySource = try swiftDepsStr
-            .map({ try VirtualPath(path: $0) })
-            .map(DependencySource.init)
+          guard let dependencySource = try swiftDepsStr
+                  .map({ try VirtualPath(path: $0) })
+                  .map(DependencySource.init)
+          else {
+            throw ReadError.unknownDependencySourceExtension
+          }
           self.finalize(node: Node(key: key,
                                    fingerprint: fingerprint,
                                    dependencySource: dependencySource))
diff --git a/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraphParts/DependencySource.swift b/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraphParts/DependencySource.swift
index 98c24c6e2..5bd5275ca 100644
--- a/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraphParts/DependencySource.swift
+++ b/Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraphParts/DependencySource.swift
@@ -26,14 +26,15 @@ public struct DependencySource: Hashable, CustomStringConvertible {
   }
 
   /*@_spi(Testing)*/
-  public init(_ file: VirtualPath) {
+  /// Returns nil if cannot be a source
+  public init?(_ file: VirtualPath) {
     let ext = file.extension
-    let typeIfExpected =
+    guard let type =
       ext == FileType.swiftDeps  .rawValue ? FileType.swiftDeps :
       ext == FileType.swiftModule.rawValue ? FileType.swiftModule
       : nil
-    guard let type = typeIfExpected else {
-      fatalError("unexpected dependencySource extension: \(String(describing: ext))")
+    else {
+      return nil
     }
     self.init(TypedVirtualPath(file: file, type: type))
   }
@@ -48,7 +49,7 @@ public struct DependencySource: Hashable, CustomStringConvertible {
 // MARK: - mocking
 extension DependencySource {
   /*@_spi(Testing)*/ public init(mock i: Int) {
-    self.init(try! VirtualPath(path: String(i) + "." + FileType.swiftDeps.rawValue))
+    self.init(try! VirtualPath(path: String(i) + "." + FileType.swiftDeps.rawValue))!
   }
 
   /*@_spi(Testing)*/ public var mockID: Int {
diff --git a/Tests/SwiftDriverTests/IncrementalCompilationTests.swift b/Tests/SwiftDriverTests/IncrementalCompilationTests.swift
index 7b8b00ac5..f4065a173 100644
--- a/Tests/SwiftDriverTests/IncrementalCompilationTests.swift
+++ b/Tests/SwiftDriverTests/IncrementalCompilationTests.swift
@@ -68,7 +68,7 @@ final class NonincrementalCompilationTests: XCTestCase {
   func testReadBinarySourceFileDependencyGraph() throws {
     let absolutePath = try XCTUnwrap(Fixture.fixturePath(at: RelativePath("Incremental"),
                                                          for: "main.swiftdeps"))
-    let dependencySource = DependencySource(VirtualPath.absolute(absolutePath))
+    let dependencySource = DependencySource(VirtualPath.absolute(absolutePath))!
     let graph = try XCTUnwrap(
       try SourceFileDependencyGraph(
         contentsOf: dependencySource,
@@ -117,7 +117,7 @@ final class NonincrementalCompilationTests: XCTestCase {
                                                          for: "hello.swiftdeps"))
     let graph = try XCTUnwrap(
       try SourceFileDependencyGraph(
-        contentsOf: DependencySource(VirtualPath.absolute(absolutePath)),
+        contentsOf: DependencySource(VirtualPath.absolute(absolutePath))!,
         on: localFileSystem))
     XCTAssertEqual(graph.majorVersion, 1)
     XCTAssertEqual(graph.minorVersion, 0)
@@ -171,7 +171,7 @@ final class NonincrementalCompilationTests: XCTestCase {
     let data = try localFileSystem.readFileContents(absolutePath)
     let graph = try XCTUnwrap(
       try SourceFileDependencyGraph(data: data,
-                                    from: DependencySource(.absolute(absolutePath)),
+                                    from: DependencySource(.absolute(absolutePath))!,
                                     fromSwiftModule: true))
     XCTAssertEqual(graph.majorVersion, 1)
     XCTAssertEqual(graph.minorVersion, 0)
@@ -822,7 +822,8 @@ class CrossModuleIncrementalBuildTests: XCTestCase {
 
       let sourcePath = path.appending(component: "main.swiftdeps")
       let data = try localFileSystem.readFileContents(sourcePath)
-      let graph = try XCTUnwrap(SourceFileDependencyGraph(data: data, from: DependencySource(.absolute(sourcePath)), fromSwiftModule: false))
+      let graph = try XCTUnwrap(SourceFileDependencyGraph(data: data, from: DependencySource(.absolute(sourcePath))!,
+                                                          fromSwiftModule: false))
       XCTAssertEqual(graph.majorVersion, 1)
       XCTAssertEqual(graph.minorVersion, 0)
       graph.verify()