diff --git a/src/parse.y b/src/parse.y
index 885d055d..c2214071 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -11,15 +11,16 @@ struct parser_control {
   pic_state *pic;
   pic_value value;
   bool incomp;
+  int yynerrs;
 };
 
 void init_scanner(const char *);
 void final_scanner(void);
 
 void yyerror(struct parser_control *, const char *);
-int yylex(struct parser_control *);
 %}
 
+%pure_parser
 %parse-param {struct parser_control *p}
 %lex-param {struct parser_control *p}
 
@@ -118,6 +119,7 @@ void
 yyerror(struct parser_control *p, const char *msg)
 {
   puts(msg);
+  p->yynerrs++;
 }
 
 bool
@@ -127,12 +129,13 @@ pic_parse(pic_state *pic, const char *str, pic_value *v)
 
   p.pic = pic;
   p.incomp = false;
+  p.yynerrs = 0;
 
   init_scanner(str);
   yyparse(&p);
   final_scanner();
 
-  if (yynerrs > 0) {
+  if (p.yynerrs > 0) {
     p.value = pic_undef_value();
   }
 
diff --git a/src/scan.l b/src/scan.l
index c9b55f74..3881b3f4 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -9,11 +9,12 @@ struct parser_control {
   pic_state *pic;
   pic_value value;
   bool incomp;
+  int yynerrs;
 };
 
 static pic_value new_escaped_string(pic_state *, const char *);
 
-#define YY_DECL int yylex (struct parser_control *p)
+#define YY_DECL int yylex (YYSTYPE *yylvalp, struct parser_control *p)
 %}
 
 %option noinput
@@ -43,11 +44,11 @@ string_elem	[^\"\\]|"\\\""|"\\\\"
 "("		return tLPAREN;
 ")"		return tRPAREN;
 "'"		return tQUOTE;
-{boolean}	{ yylval.datum = pic_bool_value(strcmp(yytext, "#t") == 0 || strcmp(yytext, "#true") == 0); return tBOOLEAN; }
-{real} 		{ yylval.datum = pic_float_value(atof(yytext)); return tNUMBER; }
-{identifier}	{ yylval.datum = pic_intern_cstr(p->pic, yytext); return tSYMBOL; }
+{boolean}	{ yylvalp->datum = pic_bool_value(strcmp(yytext, "#t") == 0 || strcmp(yytext, "#true") == 0); return tBOOLEAN; }
+{real} 		{ yylvalp->datum = pic_float_value(atof(yytext)); return tNUMBER; }
+{identifier}	{ yylvalp->datum = pic_intern_cstr(p->pic, yytext); return tSYMBOL; }
 {string}	{
-		  yylval.datum = new_escaped_string(p->pic, yytext);
+		  yylvalp->datum = new_escaped_string(p->pic, yytext);
 		  return tSTRING;
 		}