diff --git a/pandatool/src/xfile/xLexer.lxx b/pandatool/src/xfile/xLexer.lxx
index d71be462ba..b3c963fcaa 100644
--- a/pandatool/src/xfile/xLexer.lxx
+++ b/pandatool/src/xfile/xLexer.lxx
@@ -604,3 +604,8 @@ SEPARATOR            [ \t;,]+
 }
 
   
+. {
+  // Any other character is invalid.
+  accept();
+  xyyerror("Invalid character '" + string(xyytext) + "'.");
+}