diff --git a/include/picrin/value.h b/include/picrin/value.h
index fc627138..e752f36a 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -69,5 +69,6 @@ pic_value pic_float_value(double);
 #define pic_nil_p(v) ((v).type == PIC_VTYPE_NIL)
 #define pic_true_p(v) ((v).type == PIC_VTYPE_TRUE)
 #define pic_false_p(v) ((v).type == PIC_VTYPE_FALSE)
+#define pic_undef_p(v) ((v).type == PIC_VTYPE_UNDEF)
 
 #endif
diff --git a/src/main.c b/src/main.c
index 36e14f4e..1e24ed3f 100644
--- a/src/main.c
+++ b/src/main.c
@@ -3,6 +3,8 @@
 #include "picrin.h"
 
 #if PIC_ENABLE_READLINE
+# include <string.h>
+# include <stdlib.h>
 # include <readline/readline.h>
 # include <readline/history.h>
 #endif
@@ -84,6 +86,11 @@ main()
     printf("]\n");
 #endif
 
+    if (pic_undef_p(v)) {
+      pic_gc_arena_restore(pic, ai);
+      continue;
+    }
+
     /* eval */
     proc = pic_codegen(pic, v, pic->global_env);
     v = pic_run(pic, proc, pic_nil_value());
diff --git a/src/parse.y b/src/parse.y
index 5582c769..b730ee12 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -4,6 +4,8 @@
 
 #include "picrin.h"
 
+#define YYERROR_VERBOSE 1
+
 struct parser_control {
   pic_state *pic;
   pic_value value;
@@ -29,7 +31,7 @@ struct parser_control {
 program
 	:
 	{
-	  p->value = pic_nil_value(p->pic);
+	  p->value = pic_undef_value(p->pic);
 	}
 	| datum
 	{
@@ -95,7 +97,6 @@ int
 yyerror(struct parser_control *p, const char *msg)
 {
   puts(msg);
-  abort();
 }
 
 pic_value
@@ -109,5 +110,9 @@ pic_parse(pic_state *pic, const char *str)
   yyparse(&p);
   yylex_destroy();
 
+  if (yynerrs > 0) {
+    p.value = pic_undef_value();
+  }
+
   return p.value;
 }