diff --git a/src/cc65/preproc.c b/src/cc65/preproc.c
index 6de50d0b5..4af37a4b5 100644
--- a/src/cc65/preproc.c
+++ b/src/cc65/preproc.c
@@ -399,6 +399,8 @@ static void CopyQuotedString (StrBuf* Target)
     if (CurC != '\0') {
         SB_AppendChar (Target, CurC);
         NextChar ();
+    } else {
+        PPWarning ("Missing terminating %c character", Quote);
     }
 }