diff --git a/codegen/Cargo.toml b/codegen/Cargo.toml
index 1368aea4..b838a605 100644
--- a/codegen/Cargo.toml
+++ b/codegen/Cargo.toml
@@ -19,7 +19,7 @@ rocket = { version = "0.3.0", path = "../lib/" }
 log = "0.3"
 
 [dev-dependencies]
-compiletest_rs = "0.2"
+compiletest_rs = "0.2.9"
 
 [build-dependencies]
 yansi = "0.3"
diff --git a/codegen/src/decorators/derive_form.rs b/codegen/src/decorators/derive_form.rs
index ae02de85..5c2a6ba8 100644
--- a/codegen/src/decorators/derive_form.rs
+++ b/codegen/src/decorators/derive_form.rs
@@ -122,6 +122,18 @@ pub fn from_form_derive(ecx: &mut ExtCtxt, span: Span, meta_item: &MetaItem,
     trait_def.expand(ecx, meta_item, annotated, push);
 }
 
+fn is_valid_field_name(name: &str) -> bool {
+    // The HTML5 spec (4.10.18.1) says 'isindex' is not allowed.
+    if name == "isindex" || name.is_empty() {
+        return false
+    }
+
+    // We allow all visible ASCII characters except '&', '=', and '?' since we
+    // use those as control characters for parsing.
+    name.chars()
+        .all(|c| (c >= ' ' && c <= '~') && c != '&' && c != '=' && c != '?')
+}
+
 pub fn extract_field_ident_name(ecx: &ExtCtxt, struct_field: &StructField)
         -> (Ident, String, Span) {
     let ident = match struct_field.ident {
@@ -168,8 +180,10 @@ pub fn extract_field_ident_name(ecx: &ExtCtxt, struct_field: &StructField)
 
     let name = inner_item.value_str().unwrap().as_str().to_string();
     let sp = inner_item.span.shorten_upto(name.len() + 2);
-    if !is_valid_ident(&name) {
-        ecx.span_err(sp, "invalid form field identifier");
+    if !is_valid_field_name(&name) {
+        ecx.struct_span_err(sp, "invalid form field name")
+            .help("field names be visible ASCII characters without '&', '=', or '?'")
+            .emit();
     }
 
     (ident, name, sp)
diff --git a/codegen/tests/compile-fail/form-field-attr.rs b/codegen/tests/compile-fail/form-field-attr.rs
index 5ccd45c6..f6118707 100644
--- a/codegen/tests/compile-fail/form-field-attr.rs
+++ b/codegen/tests/compile-fail/form-field-attr.rs
@@ -84,7 +84,7 @@ struct MyForm10 {
 
 #[derive(FromForm)]
 struct MyForm11 {
-    #[form(field = "hello world")]
+    #[form(field = "hello&world")]
     //~^ ERROR: invalid form field
     first: String,
 }
@@ -95,3 +95,17 @@ struct MyForm12 {
     //~^ ERROR: invalid form field
     first: String,
 }
+
+#[derive(FromForm)]
+struct MyForm13 {
+    #[form(field = "?")]
+    //~^ ERROR: invalid form field
+    first: String,
+}
+
+#[derive(FromForm)]
+struct MyForm14 {
+    #[form(field = "")]
+    //~^ ERROR: invalid form field
+    first: String,
+}
diff --git a/codegen/tests/run-pass/form-field-rename.rs b/codegen/tests/run-pass/form-field-rename.rs
index 83dcef66..ec99d7a8 100644
--- a/codegen/tests/run-pass/form-field-rename.rs
+++ b/codegen/tests/run-pass/form-field-rename.rs
@@ -17,6 +17,8 @@ struct Form {
     field_type: isize,
     #[form(field = "DOUBLE")]
     double: String,
+    #[form(field = "a.b")]
+    dot: isize,
 }
 
 fn parse<'f, T: FromForm<'f>>(string: &'f str, strict: bool) -> Option<T> {
@@ -36,7 +38,7 @@ fn parse_strict<'f, T: FromForm<'f>>(string: &'f str) -> Option<T> {
 fn main() {
     let form_string = &[
         "single=100", "camelCase=helloThere", "TitleCase=HiHi", "type=-2",
-        "DOUBLE=bing_bong"
+        "DOUBLE=bing_bong", "a.b=123",
     ].join("&");
 
     let form: Option<Form> = parse_strict(&form_string);
@@ -45,12 +47,13 @@ fn main() {
         camel_case: "helloThere".into(),
         title_case: "HiHi".into(),
         field_type: -2,
-        double: "bing_bong".into()
+        double: "bing_bong".into(),
+        dot: 123,
     }));
 
     let form_string = &[
         "single=100", "camel_case=helloThere", "TitleCase=HiHi", "type=-2",
-        "DOUBLE=bing_bong"
+        "DOUBLE=bing_bong", "dot=123",
     ].join("&");
 
     let form: Option<Form> = parse_strict(&form_string);
diff --git a/codegen/tests/tests.rs b/codegen/tests/tests.rs
index 3f813710..52f4d8c9 100644
--- a/codegen/tests/tests.rs
+++ b/codegen/tests/tests.rs
@@ -3,7 +3,7 @@ extern crate compiletest_rs as compiletest;
 use std::path::PathBuf;
 
 fn run_mode(mode: &'static str) {
-    let mut config = compiletest::default_config();
+    let mut config = compiletest::Config::default();
     let cfg_mode = mode.parse().expect("Invalid mode");
 
     config.mode = cfg_mode;