diff --git a/src/cc65/preproc.c b/src/cc65/preproc.c
index f1328c4d0..a49fade20 100644
--- a/src/cc65/preproc.c
+++ b/src/cc65/preproc.c
@@ -400,6 +400,11 @@ static void OldStyleComment (void)
 static void NewStyleComment (void)
 /* Remove a new style C comment from line. */
 {
+    /* Diagnose if this is unsupported */
+    if (IS_Get (&Standard) < STD_C99) {
+        PPError ("C++ style comments are not allowed in C89");
+    }
+
     /* Beware: Because line continuation chars are handled when reading
     ** lines, we may only skip until the end of the source line, which
     ** may not be the same as the end of the input line. The end of the
@@ -432,7 +437,7 @@ static int SkipWhitespace (int SkipLines)
         } else if (CurC == '/' && NextC == '*') {
             OldStyleComment ();
             Skipped = 1;
-        } else if (IS_Get (&Standard) >= STD_C99 && CurC == '/' && NextC == '/') {
+        } else if (CurC == '/' && NextC == '/') {
             NewStyleComment ();
             Skipped = 1;
         } else if (CurC == '\0' && SkipLines) {
@@ -1004,6 +1009,7 @@ static void DoDefine (void)
                 if (CurC != '.' || NextC != '.') {
                     PPError ("'...' expected");
                     ClearLine ();
+                    FreeMacro (M);
                     return;
                 }
                 NextChar ();
@@ -1043,8 +1049,9 @@ static void DoDefine (void)
 
         /* Check for a right paren and eat it if we find one */
         if (CurC != ')') {
-            PPError ("')' expected");
+            PPError ("')' expected for macro definition");
             ClearLine ();
+            FreeMacro (M);
             return;
         }
         NextChar ();
@@ -1558,7 +1565,7 @@ static void TranslationPhase3 (StrBuf* Source, StrBuf* Target)
             } else if (CurC == '/' && NextC == '*') {
                 OldStyleComment ();
                 HasWhiteSpace = 1;
-            } else if (IS_Get (&Standard) >= STD_C99 && CurC == '/' && NextC == '/') {
+            } else if (CurC == '/' && NextC == '/') {
                 NewStyleComment ();
                 HasWhiteSpace = 1;
             } else {