diff --git a/lisp.c b/lisp.c
index 931b8df..aa5c11b 100644
--- a/lisp.c
+++ b/lisp.c
@@ -1,4 +1,5 @@
 #include <ctype.h>
+#include <setjmp.h>
 #include <stdarg.h>
 #include <stdbool.h>
 #include <stdint.h>
@@ -14,6 +15,25 @@
 #define unexpected(exp, act, ...) \
     error("expected %s but got " act, exp __VA_OPT__(,) __VA_ARGS__)
 
+static jmp_buf jmp_runtime_error;
+static char errmsg[BUFSIZ];
+
+ATTR_NORETURN
+static void runtime_error(const char *fmt, ...)
+{
+    va_list ap;
+    va_start(ap, fmt);
+    snprintf(errmsg, sizeof(errmsg), "error: ");
+    vsnprintf(errmsg, sizeof(errmsg), fmt, ap);
+
+    longjmp(jmp_runtime_error, Qundef);
+}
+
+const char *error_message(void)
+{
+    return errmsg;
+}
+
 typedef enum {
 // immediate
     TYPE_BOOL,
@@ -338,7 +358,7 @@ static const char *unintern(Symbol sym)
 {
     const char *name = name_nth(symbol_names, (long) sym);
     if (name == NULL)
-        error("symbol %lu not found", sym);
+        runtime_error("symbol %lu not found", sym);
     return name;
 }
 
@@ -428,7 +448,7 @@ static Token get_token(Parser *p)
         ungetc(c, p->in);
         return get_token_ident(p);
     }
-    error("got unexpected char '%c'", c);
+    runtime_error("got unexpected char '%c'", c);
 }
 
 static void unget_token(Parser *p, Token t)
@@ -556,8 +576,8 @@ static void expect_arity(long expected, long actual)
 {
     if (expected < 0 || expected == actual)
         return;
-    error("wrong number of arguments: expected %ld but got %ld",
-          expected, actual);
+    runtime_error("wrong number of arguments: expected %ld but got %ld",
+                  expected, actual);
 }
 
 Value apply(Value *env, Value func, Value vargs)
@@ -565,7 +585,7 @@ Value apply(Value *env, Value func, Value vargs)
     static const long FUNCARG_MAX = 7;
 
     long n = FUNCTION(func)->arity;
-    if (n > FUNCARG_MAX)
+    if (n > FUNCARG_MAX) // fatal; wrong at define_function()
         error("arguments too long: max is %ld but got %ld", FUNCARG_MAX, n);
     expect_arity(n, length(vargs));
 
@@ -637,7 +657,9 @@ static Value define_function(Value *env, const char *name, CFunc cfunc, long ari
 static Value lookup(Value env, Value name)
 {
     Value found = alist_find(env, name);
-    return found == Qnil ? Qundef : cdr(found);
+    if (found == Qnil)
+        runtime_error("variable %s not found", value_to_string(name));
+    return cdr(found);
 }
 
 Value eval_string(const char *in)
@@ -703,6 +725,8 @@ Value load(FILE *in)
 {
     Value env = toplevel_environment;
     Value last = Qnil;
+    if (setjmp(jmp_runtime_error) != 0)
+        return Qundef;
     for (Value v = parse(in); v != Qnil; v = cdr(v))
         last = ieval(&env, car(v));
     return last;
@@ -821,8 +845,8 @@ static void expect_type(Type expected, Value v, const char *header)
     Type t = value_typeof(v);
     if (t == expected)
         return;
-    error("%s: type error: expected %s but got %s",
-          header, TYPE_NAMES[expected], TYPE_NAMES[t]);
+    runtime_error("%s: type error: expected %s but got %s",
+                  header, TYPE_NAMES[expected], TYPE_NAMES[t]);
 }
 
 static Value builtin_add(Value args)
@@ -839,7 +863,7 @@ static Value builtin_add(Value args)
 static Value builtin_sub(Value args)
 {
     if (args == Qnil)
-        error("wrong number of arguments: expected 1 or more but got 0");
+        runtime_error("wrong number of arguments: expected 1 or more but got 0");
     Value rest = cdr(args);
     int64_t y = 0;
     if (rest == Qnil)
@@ -871,7 +895,7 @@ static Value builtin_mul(Value args)
 static Value builtin_div(Value args)
 {
     if (args == Qnil)
-        error("wrong number of arguments: expected 1 or more but got 0");
+        runtime_error("wrong number of arguments: expected 1 or more but got 0");
     Value rest = cdr(args);
     int64_t y = 1;
     if (rest == Qnil)
diff --git a/lisp.h b/lisp.h
index 55367b5..c2f26b3 100644
--- a/lisp.h
+++ b/lisp.h
@@ -76,4 +76,6 @@ ATTR_MALLOC char *stringify(Value v);
 Value parse_string(const char *in);
 Value parse_expr_string(const char *in);
 
+const char *error_message(void);
+
 #endif
diff --git a/test_lisp.c b/test_lisp.c
index 6ddef46..908a8b7 100644
--- a/test_lisp.c
+++ b/test_lisp.c
@@ -5,11 +5,6 @@
 
 #include "lisp.h"
 
-Test(lisp, nil) {
-    Value a = Qnil;
-    cr_assert(value_is_nil(a));
-}
-
 #define assert_stringify(expected, v) do { \
         char *s = stringify(v); \
         cr_assert_str_eq(expected, s); \
@@ -27,6 +22,16 @@ Test(lisp, nil) {
 #define V(x) \
     _Generic(x, int: value_of_int(x), const char *: value_of_symbol, Value: x)
 
+#define assert_runtime_error(v, pattern) do { \
+        cr_assert_eq(Qundef, v); \
+        cr_assert_not_null(strstr(error_message(), pattern)); \
+    } while (0)
+
+Test(lisp, nil) {
+    Value a = Qnil;
+    cr_assert(value_is_nil(a));
+}
+
 Test(lisp, printing) {
     assert_stringify("#t", Qtrue);
     assert_stringify("#f", Qfalse);
@@ -125,10 +130,10 @@ Test(lisp, eval_arithmetic_expr) {
 
 Test(lisp, unbound_variable) {
     Value v = eval_string("x");
-    cr_assert_eq(v, Qundef);
+    assert_runtime_error(v, "x not found");
 
     v = eval_string("(+ x 2)");
-    cr_assert_eq(v, Qundef);
+    assert_runtime_error(v, "x not found");
 }
 
 Test(lisp, true_false) {