diff --git a/include/cyclone/runtime.h b/include/cyclone/runtime.h
index 06793f1a..d5fbdc19 100644
--- a/include/cyclone/runtime.h
+++ b/include/cyclone/runtime.h
@@ -836,6 +836,7 @@ object Cyc_set_cell(void *, object l, object val);
 object Cyc_set_car(void *, object l, object val);
 object Cyc_set_cdr(void *, object l, object val);
 object Cyc_length(void *d, object l);
+object Cyc_length_unsafe(void *d, object l);
 object Cyc_list2vector(void *data, object cont, object l);
 object Cyc_list2string(void *d, object cont, object lst);
 object Cyc_list(void *data, int argc, object cont, ...);
diff --git a/runtime.c b/runtime.c
index cea73af4..fffc4c9a 100644
--- a/runtime.c
+++ b/runtime.c
@@ -2002,6 +2002,16 @@ object Cyc_length(void *data, object l)
   return obj_int2obj(len);
 }
 
+object Cyc_length_unsafe(void *data, object l)
+{
+  int len = 0;
+  while (l != NULL) {
+    l = cdr(l);
+    len++;
+  }
+  return obj_int2obj(len);
+}
+
 char *int_to_binary(char *b, int x)
 {
   unsigned int i = 0x80000000, leading_zeros = 1;
diff --git a/scheme/cyclone/primitives.sld b/scheme/cyclone/primitives.sld
index 480dea15..b7850758 100644
--- a/scheme/cyclone/primitives.sld
+++ b/scheme/cyclone/primitives.sld
@@ -646,7 +646,10 @@
          ((eq? p 'Cyc-compilation-environment) "Cyc_compilation_environment")
          ((eq? p 'command-line-arguments) "Cyc_command_line_arguments")
          ((eq? p 'system)         "Cyc_system")
-         ((eq? p 'length)        "Cyc_length")
+         ((eq? p 'length)        
+          (if emit-unsafe
+              "Cyc_length_unsafe"
+              "Cyc_length"))
          ((eq? p 'set-car!)      "Cyc_set_car")
          ((eq? p 'set-cdr!)      "Cyc_set_cdr")
          ((eq? p 'eq?)           "Cyc_eq")