diff --git a/src/ca65/error.c b/src/ca65/error.c
index 53e4bc5c0..7592c4557 100644
--- a/src/ca65/error.c
+++ b/src/ca65/error.c
@@ -81,7 +81,7 @@ static void VPrintMsg (const FilePos* Pos, const char* Desc,
     SB_Terminate (&Msg);
 
     /* Format the message header */
-    SB_Printf (&S, "%s(%lu): %s: ",
+    SB_Printf (&S, "%s(%u): %s: ",
                SB_GetConstBuf (GetFileName (Pos->Name)),
                Pos->Line,
                Desc);