diff --git a/lib/Curl.js b/lib/Curl.js
index 1834e4d..6253d1e 100644
--- a/lib/Curl.js
+++ b/lib/Curl.js
@@ -12,9 +12,9 @@
     return this.options[option_id] = value;
   };
 
-  Curl.prototype.setopt = function(ooption, value) {
+  Curl.prototype.setopt = function(option_name, value) {
     var option, option_id;
-    option = ooption.toUpperCase();
+    option = option_name.toUpperCase();
     if ((option_id = Curl.user_options[option]) != null) {
       this.setopt_user_(option_id, value);
     } else if ((option_id = Curl.slist_options[option]) != null) {
@@ -22,6 +22,9 @@
     } else if ((option_id = Curl.integer_options[option]) != null) {
       this.setopt_int_(option_id, value >> 0);
     } else if ((option_id = Curl.string_options[option]) != null) {
+      if (value == null) {
+        throw new Error("Cannot set option " + option_name + " to " + value + ".");
+      }
       this.setopt_str_(option_id, value.toString());
     } else {
       throw new Error("unsupported option " + option);
diff --git a/lib/Curl.toffee b/lib/Curl.toffee
index 0305a3a..69185e3 100644
--- a/lib/Curl.toffee
+++ b/lib/Curl.toffee
@@ -6,8 +6,8 @@ catch e
 Curl::setopt_user_ = (option_id, value) ->
   @options[option_id] = value
 
-Curl::setopt = (ooption, value) ->
-  option = ooption.toUpperCase()
+Curl::setopt = (option_name, value) ->
+  option = option_name.toUpperCase()
 
   # slist must be at the top of condition
   # the option exists in string_options too
@@ -18,6 +18,8 @@ Curl::setopt = (ooption, value) ->
   else if (option_id = Curl.integer_options[option])?
     @setopt_int_ option_id, value >> 0
   else if (option_id = Curl.string_options[option])?
+    if !value?
+      throw new Error("Cannot set option #{option_name} to #{value}.")
     @setopt_str_ option_id, value.toString()
   else
     throw new Error("unsupported option #{option}")