diff --git a/toolchain/lower/function_context.cpp b/toolchain/lower/function_context.cpp
index 7f23e9199215..ce7e0ad1229a 100644
--- a/toolchain/lower/function_context.cpp
+++ b/toolchain/lower/function_context.cpp
@@ -153,9 +153,16 @@ auto FunctionContext::GetDebugLoc(SemIR::InstId inst_id) -> llvm::DebugLoc {
     return llvm::DebugLoc();
   }
   auto loc = file_context_->GetLocForDI(inst_id);
-  CARBON_CHECK(loc.filename == di_subprogram_->getFile()->getFilename(),
-               "Instructions located in a different file from their "
-               "enclosing function aren't handled yet");
+  if (loc.filename != di_subprogram_->getFile()->getFilename()) {
+    // Location is from a different file. We can't represent that directly
+    // within the scope of this function's subprogram, and we don't want to
+    // generate a new subprogram, so just discard the location information. This
+    // happens for thunks when emitting the portion of the thunk that is
+    // duplicated from the original signature.
+    //
+    // TODO: Handle this case better.
+    return llvm::DebugLoc();
+  }
   return llvm::DILocation::get(builder_.getContext(), loc.line_number,
                                loc.column_number, di_subprogram_);
 }
diff --git a/toolchain/lower/testdata/impl/import_thunk.carbon b/toolchain/lower/testdata/impl/import_thunk.carbon
index e4eb51ccdcdd..4cf5e6a97841 100644
--- a/toolchain/lower/testdata/impl/import_thunk.carbon
+++ b/toolchain/lower/testdata/impl/import_thunk.carbon
@@ -31,7 +31,7 @@ impl () as I {
   fn F(b: B) -> B { return {.b = b.b}; }
 }
 
-// --- call.carbon
+// --- call_thunk.carbon
 
 library "[[@TEST_NAME]]";
 
@@ -41,6 +41,29 @@ fn Test(a: A) -> C {
   return ().(I.F)(a);
 }
 
+// --- thunk_for_imported_interface.carbon
+
+library "[[@TEST_NAME]]";
+
+import library "thunk";
+
+class X {}
+
+impl X as I {
+  fn F(b: B) -> B { return {.b = b.b}; }
+}
+
+// --- call_thunk_for_imported_interface.carbon
+
+library "[[@TEST_NAME]]";
+
+import library "thunk";
+import library "thunk_for_imported_interface";
+
+fn Test(a: A) -> C {
+  return X.(I.F)(a);
+}
+
 // CHECK:STDOUT: ; ModuleID = 'thunk.carbon'
 // CHECK:STDOUT: source_filename = "thunk.carbon"
 // CHECK:STDOUT:
@@ -106,8 +129,8 @@ fn Test(a: A) -> C {
 // CHECK:STDOUT: !18 = distinct !DISubprogram(name: "F", linkageName: "_CF:thunk.61ea2aba74ab3bf1:I.Main", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !19 = !DILocation(line: 20, column: 3, scope: !18)
 // CHECK:STDOUT: !20 = !DILocation(line: 16, column: 8, scope: !18)
-// CHECK:STDOUT: ; ModuleID = 'call.carbon'
-// CHECK:STDOUT: source_filename = "call.carbon"
+// CHECK:STDOUT: ; ModuleID = 'call_thunk.carbon'
+// CHECK:STDOUT: source_filename = "call_thunk.carbon"
 // CHECK:STDOUT:
 // CHECK:STDOUT: define void @_CTest.Main(ptr sret({ i32 }) %return, ptr %a) !dbg !4 {
 // CHECK:STDOUT: entry:
@@ -123,9 +146,74 @@ fn Test(a: A) -> C {
 // CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
-// CHECK:STDOUT: !3 = !DIFile(filename: "call.carbon", directory: "")
+// CHECK:STDOUT: !3 = !DIFile(filename: "call_thunk.carbon", directory: "")
 // CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Test", linkageName: "_CTest.Main", scope: null, file: !3, line: 6, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 7, column: 10, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 7, column: 3, scope: !4)
+// CHECK:STDOUT: ; ModuleID = 'thunk_for_imported_interface.carbon'
+// CHECK:STDOUT: source_filename = "thunk_for_imported_interface.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @"_CF.X.Main:I.Main"(ptr sret({ i32 }) %return, ptr %b) !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %.loc9_35.1.b = getelementptr inbounds nuw { i32 }, ptr %b, i32 0, i32 0, !dbg !7
+// CHECK:STDOUT:   %.loc9_35.2 = load i32, ptr %.loc9_35.1.b, align 4, !dbg !7
+// CHECK:STDOUT:   %.loc9_37.2.b = getelementptr inbounds nuw { i32 }, ptr %return, i32 0, i32 0, !dbg !8
+// CHECK:STDOUT:   store i32 %.loc9_35.2, ptr %.loc9_37.2.b, align 4, !dbg !8
+// CHECK:STDOUT:   ret void, !dbg !9
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @"_CF:thunk.X.Main:I.Main"(ptr sret({ i32 }) %return, ptr %a) !dbg !10 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   %.loc9_19.1.temp = alloca { i32 }, align 8, !dbg !11
+// CHECK:STDOUT:   %.2.temp = alloca { i32 }, align 8
+// CHECK:STDOUT:   call void @"_CConvert.A.Main:ImplicitAs.Core"(ptr %.2.temp, ptr %a)
+// CHECK:STDOUT:   call void @"_CF.X.Main:I.Main"(ptr %.loc9_19.1.temp, ptr %.2.temp), !dbg !11
+// CHECK:STDOUT:   %.loc9_19.2.temp = alloca { i32 }, align 8, !dbg !11
+// CHECK:STDOUT:   call void @"_CConvert.B.Main:ImplicitAs.Core"(ptr %.loc9_19.2.temp, ptr %.loc9_19.1.temp), !dbg !11
+// CHECK:STDOUT:   ret void, !dbg !11
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @"_CConvert.A.Main:ImplicitAs.Core"(ptr sret({ i32 }), ptr)
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @"_CConvert.B.Main:ImplicitAs.Core"(ptr sret({ i32 }), ptr)
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!2}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !3 = !DIFile(filename: "thunk_for_imported_interface.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.X.Main:I.Main", scope: null, file: !3, line: 9, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !7 = !DILocation(line: 9, column: 34, scope: !4)
+// CHECK:STDOUT: !8 = !DILocation(line: 9, column: 28, scope: !4)
+// CHECK:STDOUT: !9 = !DILocation(line: 9, column: 21, scope: !4)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "F", linkageName: "_CF:thunk.X.Main:I.Main", scope: null, file: !3, line: 9, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !11 = !DILocation(line: 9, column: 3, scope: !10)
+// CHECK:STDOUT: ; ModuleID = 'call_thunk_for_imported_interface.carbon'
+// CHECK:STDOUT: source_filename = "call_thunk_for_imported_interface.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @_CTest.Main(ptr sret({ i32 }) %return, ptr %a) !dbg !4 {
+// CHECK:STDOUT: entry:
+// CHECK:STDOUT:   call void @"_CF:thunk.X.Main:I.Main"(ptr %return, ptr %a), !dbg !7
+// CHECK:STDOUT:   ret void, !dbg !8
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @"_CF:thunk.X.Main:I.Main"(ptr sret({ i32 }), ptr)
+// CHECK:STDOUT:
+// CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
+// CHECK:STDOUT: !llvm.dbg.cu = !{!2}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
+// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
+// CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+// CHECK:STDOUT: !3 = !DIFile(filename: "call_thunk_for_imported_interface.carbon", directory: "")
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Test", linkageName: "_CTest.Main", scope: null, file: !3, line: 7, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
+// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !7 = !DILocation(line: 8, column: 10, scope: !4)
+// CHECK:STDOUT: !8 = !DILocation(line: 8, column: 3, scope: !4)