diff --git a/src/main/java/com/squareup/phrase/Phrase.java b/src/main/java/com/squareup/phrase/Phrase.java
index f642797..49c436e 100644
--- a/src/main/java/com/squareup/phrase/Phrase.java
+++ b/src/main/java/com/squareup/phrase/Phrase.java
@@ -278,6 +278,11 @@ private KeyToken key(Token prev) {
       consume();
     }
 
+    // Disallow uppercase
+    if ((curChar >= 'A' && curChar <= 'Z')) {
+      throw new IllegalArgumentException("Uppercase Not Allowed: '" + curChar + "'");
+    }
+
     // Consume the closing '}'.
     if (curChar != '}') {
       throw new IllegalArgumentException("Missing closing brace: }");
diff --git a/src/test/java/com/squareup/phrase/PhraseTest.java b/src/test/java/com/squareup/phrase/PhraseTest.java
index b1332ac..205ec04 100644
--- a/src/test/java/com/squareup/phrase/PhraseTest.java
+++ b/src/test/java/com/squareup/phrase/PhraseTest.java
@@ -116,6 +116,14 @@ public class PhraseTest {
         .isEqualTo("Hello Eric");
   }
 
+  @Test public void putFailUppercaseNotAllowed() {
+    try{
+      from("Hello {Name}").putOptional("Name", "Eric").format().toString();
+      fail("Expected IllegalArgumentException");
+    } catch (IllegalArgumentException e) {
+    }
+  }
+
   private Phrase gender = from("{gender}");
 
   @Test