diff --git a/src/main/scala/millfork/compiler/mos/MosStatementCompiler.scala b/src/main/scala/millfork/compiler/mos/MosStatementCompiler.scala
index 2faa8575..ad557b0a 100644
--- a/src/main/scala/millfork/compiler/mos/MosStatementCompiler.scala
+++ b/src/main/scala/millfork/compiler/mos/MosStatementCompiler.scala
@@ -198,11 +198,15 @@ object MosStatementCompiler extends AbstractStatementCompiler[AssemblyLine] {
               stackPointerFixBeforeReturn(ctx) ++
                 List(AssemblyLine.discardAF(), AssemblyLine.discardXF(), AssemblyLine.discardYF()) ++ returnInstructions
             case 1 =>
-              ErrorReporting.warn("Returning without a value", ctx.options, statement.position)
+              if (statement.position.isDefined){
+                ErrorReporting.warn("Returning without a value", ctx.options, statement.position)
+              }
               stackPointerFixBeforeReturn(ctx) ++
                 List(AssemblyLine.discardXF(), AssemblyLine.discardYF()) ++ returnInstructions
             case 2 =>
-              ErrorReporting.warn("Returning without a value", ctx.options, statement.position)
+              if (statement.position.isDefined){
+                ErrorReporting.warn("Returning without a value", ctx.options, statement.position)
+              }
               stackPointerFixBeforeReturn(ctx) ++
                 List(AssemblyLine.discardYF()) ++ returnInstructions
           }
diff --git a/src/main/scala/millfork/error/ErrorReporting.scala b/src/main/scala/millfork/error/ErrorReporting.scala
index 0662821b..664463a5 100644
--- a/src/main/scala/millfork/error/ErrorReporting.scala
+++ b/src/main/scala/millfork/error/ErrorReporting.scala
@@ -3,28 +3,32 @@ package millfork.error
 import millfork.{CompilationFlag, CompilationOptions}
 import millfork.node.Position
 
+import scala.collection.mutable
+
 object ErrorReporting {
 
   var verbosity = 0
 
   var hasErrors = false
 
-  private var sourceLines: Option[IndexedSeq[String]] = None
+  private var sourceLines: mutable.Map[String, IndexedSeq[String]] = mutable.Map()
 
-  private def printErrorContext(pos: Option[Position]): Unit = {
-    if (sourceLines.isDefined && pos.isDefined) {
-      val line = sourceLines.get.apply(pos.get.line - 1)
-      val column = pos.get.column - 1
-      val margin = "       "
-      print(margin)
-      println(line)
-      print(margin)
-      print(" " * column)
-      println("^")
+  private def printErrorContext(pos: Option[Position]): Unit = synchronized {
+    pos.foreach { position =>
+        sourceLines.get(position.moduleName).foreach { lines =>
+          val line = lines.apply(pos.get.line - 1)
+          val column = pos.get.column - 1
+          val margin = "       "
+          print(margin)
+          println(line)
+          print(margin)
+          print(" " * column)
+          println("^")
+        }
     }
   }
 
-  def f(position: Option[Position]): String = position.fold("")(p => s"(${p.line}:${p.column}) ")
+  def f(position: Option[Position]): String = position.fold("")(p => s"(${p.moduleName}:${p.line}:${p.column}) ")
 
   def info(msg: String, position: Option[Position] = None): Unit = {
     if (verbosity < 0) return
@@ -94,13 +98,18 @@ object ErrorReporting {
     }
   }
 
-  def clearErrors(): Unit = {
+  def clearErrors(): Unit = synchronized {
     hasErrors = false
-    sourceLines = None
+    sourceLines.clear()
   }
 
-  def setSource(source: Option[IndexedSeq[String]]): Unit = {
-    sourceLines = source
+  def setSource(source: Option[IndexedSeq[String]]): Unit = synchronized {
+    sourceLines.clear()
+    source.foreach(sourceLines("") = _)
+  }
+
+  def addSource(filename: String, lines: IndexedSeq[String]): Unit = synchronized {
+    sourceLines(filename) = lines
   }
 
 }
\ No newline at end of file
diff --git a/src/main/scala/millfork/node/Node.scala b/src/main/scala/millfork/node/Node.scala
index 90b769db..a032952a 100644
--- a/src/main/scala/millfork/node/Node.scala
+++ b/src/main/scala/millfork/node/Node.scala
@@ -4,7 +4,7 @@ import millfork.assembly.mos.{AddrMode, Opcode}
 import millfork.assembly.z80.{ZOpcode, ZRegisters}
 import millfork.env.{Constant, ParamPassingConvention, Type}
 
-case class Position(filename: String, line: Int, column: Int, cursor: Int)
+case class Position(moduleName: String, line: Int, column: Int, cursor: Int)
 
 sealed trait Node {
   var position: Option[Position] = None
diff --git a/src/main/scala/millfork/parser/AbstractSourceLoadingQueue.scala b/src/main/scala/millfork/parser/AbstractSourceLoadingQueue.scala
index 4905375f..56c29ca7 100644
--- a/src/main/scala/millfork/parser/AbstractSourceLoadingQueue.scala
+++ b/src/main/scala/millfork/parser/AbstractSourceLoadingQueue.scala
@@ -59,7 +59,9 @@ abstract class AbstractSourceLoadingQueue[T](val initialFilenames: List[String],
     val path = Paths.get(filename)
     val parentDir = path.toFile.getAbsoluteFile.getParent
     val (src, featureConstants) = Preprocessor(options, Files.readAllLines(path, StandardCharsets.UTF_8).toIndexedSeq)
-    val parser = createParser(filename, src, parentDir, featureConstants)
+    val shortFileName = path.getFileName.toString
+    val parser = createParser(shortFileName, src, parentDir, featureConstants)
+    ErrorReporting.addSource(shortFileName, src.lines.toIndexedSeq)
     parser.toAst match {
       case Success(prog, _) =>
         parsedModules.synchronized {
diff --git a/src/main/scala/millfork/parser/MfParser.scala b/src/main/scala/millfork/parser/MfParser.scala
index 53a0abc9..16b3b54d 100644
--- a/src/main/scala/millfork/parser/MfParser.scala
+++ b/src/main/scala/millfork/parser/MfParser.scala
@@ -13,11 +13,11 @@ import millfork.{CompilationFlag, CompilationOptions, SeparatedList}
 /**
   * @author Karol Stasiak
   */
-abstract class MfParser[T](filename: String, input: String, currentDirectory: String, options: CompilationOptions, featureConstants: Map[String, Long]) {
+abstract class MfParser[T](fileId: String, input: String, currentDirectory: String, options: CompilationOptions, featureConstants: Map[String, Long]) {
 
   import MfParser._
 
-  var lastPosition = Position(filename, 1, 1, 0)
+  var lastPosition = Position(fileId, 1, 1, 0)
   var lastLabel = ""
 
   def toAst: Parsed[Program] = program.parse(input + "\n\n\n")
@@ -33,7 +33,7 @@ abstract class MfParser[T](filename: String, input: String, currentDirectory: St
     }
     val columnNumber = i - lineStarts(lineNumber)
     lineNumber += 1
-    val newPosition = Position(filename, lineNumber, columnNumber, i)
+    val newPosition = Position(fileId, lineNumber, columnNumber, i)
     if (newPosition.cursor > lastPosition.cursor) {
       lastPosition = newPosition
       lastLabel = label