diff --git a/lib/chibi/ast.c b/lib/chibi/ast.c
index 6cdfdb9b..fdb6d9be 100644
--- a/lib/chibi/ast.c
+++ b/lib/chibi/ast.c
@@ -292,6 +292,11 @@ static sexp sexp_string_contains (sexp ctx sexp_api_params(self, n), sexp x, sex
   return res ? sexp_make_fixnum(res-sexp_string_data(x)) : SEXP_FALSE;
 }
 
+static sexp sexp_error_string (sexp ctx sexp_api_params(self, n), sexp x) {
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, x);
+  return sexp_c_string(ctx, strerror(sexp_unbox_fixnum(x)), -1);
+}
+
 #define sexp_define_type(ctx, name, tag) \
   sexp_env_define(ctx, env, sexp_intern(ctx, name, -1), sexp_type_by_index(ctx, tag));
 
@@ -397,5 +402,6 @@ sexp sexp_init_library (sexp ctx sexp_api_params(self, n), sexp env) {
   sexp_define_foreign_opt(ctx, env, "integer->immediate", 2, sexp_integer_to_immediate, SEXP_FALSE);
   sexp_define_foreign(ctx, env, "gc", 0, sexp_gc_op);
   sexp_define_foreign(ctx, env, "string-contains", 2, sexp_string_contains);
+  sexp_define_foreign(ctx, env, "integer->error-string", 1, sexp_error_string);
   return SEXP_VOID;
 }
diff --git a/lib/chibi/ast.module b/lib/chibi/ast.module
index 097a851d..0ea29740 100644
--- a/lib/chibi/ast.module
+++ b/lib/chibi/ast.module
@@ -32,7 +32,7 @@
    port-line port-line-set!
    type-name type-cpl type-parent type-slots
    object-size integer->immediate gc
-   string-contains)
+   string-contains integer->error-string)
   (import (scheme))
   (include-shared "ast")
   (include "ast.scm"))