diff --git a/fluent-syntax/src/errors.js b/fluent-syntax/src/errors.js
index 0d124ae01..63f5186dc 100644
--- a/fluent-syntax/src/errors.js
+++ b/fluent-syntax/src/errors.js
@@ -23,7 +23,7 @@ function getErrorMessage(code, args) {
     }
     case 'E0005': {
       const [id] = args;
-      return `Expected entry "${id}" to have a value, attributes or tags`;
+      return `Expected entry "${id}" to have a value or attributes`;
     }
     case 'E0006': {
       const [field] = args;
diff --git a/fluent-syntax/src/ftlstream.js b/fluent-syntax/src/ftlstream.js
index 3fef5b179..9fc92701b 100644
--- a/fluent-syntax/src/ftlstream.js
+++ b/fluent-syntax/src/ftlstream.js
@@ -226,7 +226,7 @@ export class FTLParserStream extends ParserStream {
       this.next();
       return ret;
     }
-    throw new ParseError('E0004', 'a-zA-Z');
+    throw new ParseError('E0004', 'a-zA-Z_');
   }
 
   takeIDChar() {
@@ -243,15 +243,11 @@ export class FTLParserStream extends ParserStream {
 
   takeSymbChar() {
     const closure = ch => {
-      if (ch === undefined) {
-        return false;
-      }
-
       const cc = ch.charCodeAt(0);
       return ((cc >= 97 && cc <= 122) || // a-z
               (cc >= 65 && cc <= 90) || // A-Z
               (cc >= 48 && cc <= 57) || // 0-9
-               cc === 95 || cc === 45 || cc === 32);  // _-
+               cc === 95 || cc === 45 || cc === 32);  // _-<space>
     };
 
     return this.takeChar(closure);
diff --git a/fluent-syntax/src/parser.js b/fluent-syntax/src/parser.js
index 3d8321beb..df2bddc72 100644
--- a/fluent-syntax/src/parser.js
+++ b/fluent-syntax/src/parser.js
@@ -153,7 +153,7 @@ export default class FluentParser {
 
       ps.next();
 
-      if (ps.current() === '/') {
+      if (ps.currentIs('/')) {
         content += '\n';
         ps.next();
         ps.expectChar('/');
@@ -212,7 +212,7 @@ export default class FluentParser {
       tags = this.getTags(ps);
     }
 
-    if (pattern === undefined && attrs === undefined && tags === undefined) {
+    if (pattern === undefined && attrs === undefined) {
       throw new ParseError('E0005', id.name);
     }
 
@@ -566,7 +566,14 @@ export default class FluentParser {
 
       ps.expectChar(')');
 
-      return new AST.CallExpression(literal.id, args);
+      if (!/^[A-Z_-]+$/.test(literal.id.name)) {
+        throw new ParseError('E0008');
+      }
+
+      return new AST.CallExpression(
+        new AST.Function(literal.id.name),
+        args
+      );
     }
 
     return literal;
diff --git a/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl b/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl
index 5fbf7b777..ad7e751ab 100644
--- a/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl
+++ b/fluent-syntax/test/fixtures_behavior/attribute_expression_with_wrong_attr.ftl
@@ -1,7 +1,7 @@
 key = { foo.23 }
 
-//~ ERROR E0004, pos 12, args "a-zA-Z"
+//~ ERROR E0004, pos 12, args "a-zA-Z_"
 
 key = { foo. }
 
-//~ ERROR E0004, pos 31, args "a-zA-Z"
+//~ ERROR E0004, pos 31, args "a-zA-Z_"
diff --git a/fluent-syntax/test/fixtures_behavior/call_expression_with_bad_id.ftl b/fluent-syntax/test/fixtures_behavior/call_expression_with_bad_id.ftl
new file mode 100644
index 000000000..aa86f73fa
--- /dev/null
+++ b/fluent-syntax/test/fixtures_behavior/call_expression_with_bad_id.ftl
@@ -0,0 +1,3 @@
+key = { no-caps-name() }
+
+//~ ERROR E0008, pos 22
diff --git a/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl b/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl
index 1cf580b29..c44257e52 100644
--- a/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl
+++ b/fluent-syntax/test/fixtures_behavior/non_id_attribute_name.ftl
@@ -1,3 +1,3 @@
 key = Value
     .2 = Foo
-//~ ERROR E0004, pos 17, args "a-zA-Z"
+//~ ERROR E0004, pos 17, args "a-zA-Z_"
diff --git a/fluent-syntax/test/fixtures_behavior/unclosed_empty_placeable_error.ftl b/fluent-syntax/test/fixtures_behavior/unclosed_empty_placeable_error.ftl
index 0c86cfb18..c714e5caa 100644
--- a/fluent-syntax/test/fixtures_behavior/unclosed_empty_placeable_error.ftl
+++ b/fluent-syntax/test/fixtures_behavior/unclosed_empty_placeable_error.ftl
@@ -1,2 +1,2 @@
 bar = Bar {
-//~ ERROR E0004, pos 11, args "a-zA-Z"
+//~ ERROR E0004, pos 11, args "a-zA-Z_"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl b/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl
index 94dfcd4b5..4fc37412e 100644
--- a/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl
+++ b/fluent-syntax/test/fixtures_behavior/variant_ends_abruptly.ftl
@@ -1,3 +1,3 @@
 key = { $foo ->
     *[
-//~ ERROR E0004, pos 22, args "a-zA-Z"
+//~ ERROR E0004, pos 22, args "a-zA-Z_"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl b/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl
index c6fd0b8d1..950bf3289 100644
--- a/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl
+++ b/fluent-syntax/test/fixtures_behavior/variant_expression_empty_key.ftl
@@ -1,2 +1,2 @@
 key = { foo[] }
-//~ ERROR E0004, pos 12, args "a-zA-Z"
+//~ ERROR E0004, pos 12, args "a-zA-Z_"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl b/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl
index 139f4f108..5fb3db3e6 100644
--- a/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl
+++ b/fluent-syntax/test/fixtures_behavior/variant_starts_from_nl.ftl
@@ -1,4 +1,4 @@
 key = {
 *[one] Value
     }
-//~ ERROR E0004, pos 7, args "a-zA-Z"
+//~ ERROR E0004, pos 7, args "a-zA-Z_"
diff --git a/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl b/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl
index 0b8f9c956..51a8f1af4 100644
--- a/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl
+++ b/fluent-syntax/test/fixtures_behavior/variant_with_leading_space_in_name.ftl
@@ -1,4 +1,4 @@
 key = {
         *[  one] Foo
     }
-//~ ERROR E0004, pos 18, args "a-zA-Z"
+//~ ERROR E0004, pos 18, args "a-zA-Z_"