diff --git a/src/main.c b/src/main.c
index 211b932..2508a7f 100644
--- a/src/main.c
+++ b/src/main.c
@@ -138,6 +138,8 @@ static void parse_preamble(struct parser *p) {
 			}
 			fprintf(p->output, "\n");
 			break;
+		} else {
+			parser_fatal(p, "Name characters must be A-Z, a-z, 0-9, `-`, `_`, or `.`");
 		}
 	}
 	str_free(name);