@@ -590,11 +590,12 @@
 		void *pool = objc_autoreleasePoolPush();
 		const char *UTF8String = string.UTF8String;
 		size_t length = string.UTF8StringLength;
 		const char *colon;
 
-		if ((colon = strchr(UTF8String, ':')) == NULL)
+		if ((colon = strchr(UTF8String, ':')) == NULL ||
+		    colon - UTF8String < 1 || !OFASCIIIsAlpha(UTF8String[0]))
 			@throw [OFInvalidFormatException exception];
 
 		_percentEncodedScheme = [[[OFString
 		    stringWithUTF8String: UTF8String
 				  length: colon - UTF8String] lowercaseString]