diff --git a/src/lx/parser.act b/src/lx/parser.act
index 78254e123..f9b0b32f9 100644
--- a/src/lx/parser.act
+++ b/src/lx/parser.act
@@ -869,7 +869,19 @@
 
 		lx->free(lx->buf_opaque);
 
-		assert(ast != NULL);
+		/*
+		 * This can happen when the first token is TOK_UNKNOWN.
+		 * SID's generated parser bails out immediately.
+		 * So we never reach <make-ast>, and never <err-syntax> about it.
+		 *
+		 * Really I wanted this handled along with the usual syntax error
+		 * case, from ## alts inside parser.sid, instead of reproducing
+		 * the same error here.
+		 */
+		if (ast == NULL) {
+			err(lex_state, "Syntax error");
+			exit(EXIT_FAILURE);
+		}
 
 		return ast;
 	}
diff --git a/src/lx/parser.c b/src/lx/parser.c
index cbc2067fd..dfcf36b8f 100644
--- a/src/lx/parser.c
+++ b/src/lx/parser.c
@@ -3364,11 +3364,23 @@ ZL1:;
 
 		lx->free(lx->buf_opaque);
 
-		assert(ast != NULL);
+		/*
+		 * This can happen when the first token is TOK_UNKNOWN.
+		 * SID's generated parser bails out immediately.
+		 * So we never reach <make-ast>, and never <err-syntax> about it.
+		 *
+		 * Really I wanted this handled along with the usual syntax error
+		 * case, from ## alts inside parser.sid, instead of reproducing
+		 * the same error here.
+		 */
+		if (ast == NULL) {
+			err(lex_state, "Syntax error");
+			exit(EXIT_FAILURE);
+		}
 
 		return ast;
 	}
 
-#line 3373 "src/lx/parser.c"
+#line 3385 "src/lx/parser.c"
 
 /* END OF FILE */
diff --git a/src/lx/parser.h b/src/lx/parser.h
index fbe704fc4..bde7c36ab 100644
--- a/src/lx/parser.h
+++ b/src/lx/parser.h
@@ -29,7 +29,7 @@
 extern void p_lx(lex_state, act_state, ast *);
 /* BEGINNING OF TRAILER */
 
-#line 877 "src/lx/parser.act"
+#line 889 "src/lx/parser.act"
 
 
 #line 36 "src/lx/parser.h"